Remove duplication of base classes for OAuth now that we depend on oauth2client.
Remove duplication of base classes for OAuth now that
we depend on oauth2client.

Reviewed in http://codereview.appspot.com/5599044/.

googleapis/oauth2client,googleapis/google-api-python-client,jonparrott/oauth2client,clancychilds/oauth2client,googleapis/google-api-python-client,jonparrott/oauth2client,clancychilds/oauth2client,google/oauth2client,google/oauth2client,googleapis/oauth2client