Add Google OAuth2.0 Api Support

¡Hola, Pablo!

Would you please review my implementation of OAuth 2.0 support for Google?

I'm having problems accessing the YouTube API 3.0 using the old GoogleApi. Whereas GoogleApi only supports OAuth10a, YouTube API 3.0 only supports OAuth 2.0.

I created this class based on FacebookApi (which also implements OAuth 2.0).
I haven't tested it yet though.
What I'm still not sure about is how to properly implement the "scope" parameter...

Saludos,
Leandro