Added OAuth2 support

I added OAuth2 support for authorization which is compatible with Google's OAuth2 library:
https://github.com/google/oauth2client

I tried to keep it simple because of all the OAuth2 scenarios and assumed the user's code would handle generating the credentials object.  I nested the import of httplib2 so that is only a requirement only if you use the OAuth2 authorization and the credentials are not authorized.

This fixes #21 and #86.

Lastly, I added some docs for OAuth but don't know if that is the right spot to put it.  Let me know if I need to change that.