Add verification if user is currently logged in to his Google account

[GoogleCodeExporter]

It is required for user to be logged in into Google account in order for 
extension to function. If user is currently no logged in extension should show 
a dialog to login.

Original issue reported on code.google.com by [email protected] on 20 Mar 2014 at 11:57

[kdidenko]

Was implemented long time ago inside a scripts/identity.js lib, even before ES6's Promises were actually supported anywhere (starting from Chrome v.32), while issue haven't been still closed. Now it's already time to:

1. Rename the file to oauth2-client.js
2. Implement separate authentication class for each service that user have to be authenticated (it's actually still only his Google Account at the moment, but more to come shortly!)
3. Refactor the code from long chains of callback based if/else into a Promise -based style
4. Test it
5. Close the issue than