Replace oauth2 with requests-oauthlib

[oauth2](https://pypi.python.org/pypi/oauth2) seems to be a dead project and does not support python 3.3. [requests-oauthlib](https://pypi.python.org/pypi/requests-oauthlib) seems to be very much alive and well according to pypi.

Replacing oauth2 with requests-oauthlib seems like a good idea.
