Adding support for Google Drive rivers

Hello David,

here is a PR for issue #4 : Add a Google Drive connection. 

Everything works well, however I encounter a bug on the settings page : the AdminRiverServices for FS, Dropbox and Google Drive return each an instance of my newly created river ... The get() method does not seem to filter River according the generic type given by the AdminService.

I've choosen the Dropboxrivers branche cause it was my starting point. Tell me if it's ok for you.

Regards,