separate api and token storage contract

[nosovk]

In current implementation we are dependent on file system, which could be not available in lots of environments.
Like: https://community.cloudflare.com/t/about-cloudflare-workers-filesystem/481642
There was a similar request for Firebase: #17

In firebase, you can get short living tempdir, but in Cloudflare you couldn't event that.

Proposed solution: migrate to interface based providers. Like:

sendpulse.init(API_USER_ID,API_SECRET,TOKEN_STORAGE,function() {
    sendpulse.listAddressBooks(console.log);
});

where TOKEN_STORAGE is an object or class, that implements simple api with two methods:
storage.get("key"), storage.set("key", value)
In that case we can provide different storage providers, without hard dependency on fs.

Or any other approach will be also ok.