Replacing Flask-OAuth with Flask-OAuthlib

Update to the Flask-OAuthlib 0.7.0 as *"Flask-OAuthlib is designed as a replacement for Flask-OAuth"* and that *"the client part of Flask-OAuthlib shares the same API as Flask-OAuth, which is pretty and simple"* (see https://flask-oauthlib.readthedocs.org/en/latest/ and https://github.com/lepture/flask-oauthlib).

If you experience déjà-vu: see https://github.com/gae-init/gae-init/pull/56 for a previous PR on this subject.
:dancer: this is soooo rare ;)