Google OAuth 2 API support.

Hello Pablo,

I have to support Google OAuth 2 login in my project. I added this API to the apis, and there are some additional changes too. First of all the grant_type is a required parameter in OAuth 2, so it could be set via API objects yet. The Google API requires some changes in request parameter handling, because they wanna use POST instead of GET. So I merged body parameters and querystring parameters, and only the Verb defines the request method. The Google access_token answer contains whitespaces between the name and the colon - I changed the regex pattern of JsonTokenExtractor to tolerate this.

Please, check my changes, and if you find them useful, then pull it back please in the main project. I tested the new version against Facebook, Live and Google via OAuth 2 interfaces.

Thanks for this great library!

Regards,
Laszlo