Add method to create a draft

Author: linkyndy

README.md

@@ -120,6 +120,24 @@ params = {
 message = gmail.send_message(**params)  # equivalent to send_message(to="you@youremail.com", sender=...)
 ```
 
+### Create a draft:
+
+```python
+from simplegmail import Gmail
+
+gmail = Gmail() # will open a browser window to ask you to log in and authenticate
+
+params = {
+  "to": "you@youremail.com",
+  "sender": "me@myemail.com",
+  "subject": "My first email",
+  "msg_html": "<h1>Woah, my first email!</h1><br />This is an HTML email.",
+  "msg_plain": "Hi\nThis is a plain text email.",
+  "signature": True  # use my account signature
+}
+draft = gmail.create_draft(**params)  # equivalent to create_draft(to="you@youremail.com", sender=...)
+```
+
 It couldn't be easier!
 
 ### Retrieving messages:

simplegmail/draft.py

@@ -13,6 +13,7 @@
 from simplegmail import label
 from simplegmail.attachment import Attachment
 from simplegmail.label import Label
+from simplegmail.message import Message
 
 
 class Draft(object):

simplegmail/gmail.py

@@ -31,6 +31,7 @@
 
 from simplegmail import label
 from simplegmail.attachment import Attachment
+from simplegmail.draft import Draft
 from simplegmail.label import Label
 from simplegmail.message import Message
 
@@ -176,6 +177,63 @@ def send_message(
             # Pass along the error
             raise error
 
+    def create_draft(
+        self,
+        sender: str,
+        to: str,
+        subject: str = '',
+        msg_html: Optional[str] = None,
+        msg_plain: Optional[str] = None,
+        cc: Optional[List[str]] = None,
+        bcc: Optional[List[str]] = None,
+        attachments: Optional[List[str]] = None,
+        signature: bool = False,
+        user_id: str = 'me'
+    ) -> Message:
+        """
+        Creates a draft.
+
+        Args:
+            sender: The email address the draft is being sent from.
+            to: The email address the draft is being sent to.
+            subject: The subject line of the email.
+            msg_html: The HTML message of the email.
+            msg_plain: The plain text alternate message of the email. This is
+                often displayed on slow or old browsers, or if the HTML message
+                is not provided.
+            cc: The list of email addresses to be cc'd.
+            bcc: The list of email addresses to be bcc'd.
+            attachments: The list of attachment file names.
+            signature: Whether the account signature should be added to the
+                draft.
+            user_id: The address of the sending account. 'me' for the
+                default address associated with the account.
+
+        Returns:
+            The Draft object representing the created draft.
+
+        Raises:
+            googleapiclient.errors.HttpError: There was an error executing the
+                HTTP request.
+
+        """
+
+        msg = {
+            'message': self._create_message(
+                sender, to, subject, msg_html, msg_plain, cc=cc, bcc=bcc,
+                attachments=attachments, signature=signature, user_id=user_id
+            )
+        }
+
+        try:
+            req = self.service.users().drafts().create(userId='me', body=msg)
+            res = req.execute()
+            return self._build_draft_from_ref(user_id, res, 'reference')
+
+        except HttpError as error:
+            # Pass along the error
+            raise error
+
     def get_unread_inbox(
         self,
         user_id: str = 'me',
@@ -849,6 +907,56 @@ def _build_message_from_ref(
                 bcc
             )
 
+    def _build_draft_from_ref(
+        self,
+        user_id: str,
+        draft_ref: dict,
+        attachments: str = 'reference'
+    ) -> Draft:
+        """
+        Creates a Draft object from a reference.
+
+        Args:
+            user_id: The username of the account the draft belongs to.
+            draft_ref: The draft reference object returned from the Gmail
+                API.
+            attachments: Accepted values are 'ignore' which completely ignores
+                all attachments, 'reference' which includes attachment
+                information but does not download the data, and 'download' which
+                downloads the attachment data to store locally. Default
+                'reference'.
+
+        Returns:
+            The Draft object.
+
+        Raises:
+            googleapiclient.errors.HttpError: There was an error executing the
+                HTTP request.
+
+        """
+
+        try:
+            # Get draft JSON
+            draft = self.service.users().drafts().get(
+                userId=user_id, id=draft_ref['id']
+            ).execute()
+
+        except HttpError as error:
+            # Pass along the error
+            raise error
+
+        else:
+            id = draft['id']
+            message = self._build_message_from_ref(user_id, draft['message'], attachments)
+
+            return Draft(
+                self.service,
+                self.creds,
+                user_id,
+                id,
+                message
+            )
+
     def _evaluate_message_payload(
         self,
         payload: dict,