Using post method for google oauth2.0 requires args in body

[FreakTheMighty]

I'm not sure if google's implementation is non-standard, but I've found that to get the access_token I need to use a "POST" instead of a command. However simply setting access_token_method to "POST" is not sufficient because "handle_oauth2_response" puts the remote_args into the url instead of the body.

This may require a separate ticket, but google also expects an additional argument "grant_type" which needs to be set to "authorization_code".

[jaimeirurzun]

Method is now configurable via access_token_method. For adding params, there is a pull request here:

#21