constructor should work with integer

```
>>> DataSize(4836071272)
>>> Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/datasize/__datasize__.py", line 97, in __new__
    raw = spec[:]
TypeError: 'int' object is not subscriptable

```