Adding idToken to Android

Based on this gist [1] I'm adding support for getting the auth oauth2 token
that Google generates after the user accepts the login.

[1] https://gist.github.com/ianbarber/5170508
Introducing idToken and oauthToken

Allow the user to configure the service so any of the 3 possibles can be reached
when the user passes webApiKey in the credentials dictionary will get a JWT token
id, if androidApiKey is set in the credentials then an off-line capable oauth2
token bearer will be generated in the oauthToken value, while if no argument is
set a simple oauth2 token will get generated.
Adding extra necessary permissions
Merge pull request #21 from manuelnaranjo/master

Adding idToken to Android