Re: oauth2 patch

Hello in this patch I have implemented OAuth2. Unfortunately I wasn't
sure how the test should look like, because I couldn't find anything in
the developer documentation, so I decided not to write one for now.

The configuration has to look like this here is an example for github:

OAUTH2_NAME = 'github'
OAUTH2_CLIENT_ID = 'secret'
OAUTH2_CLIENT_SECRET = 'secret'
OAUTH2_TOKEN_URL = 'https://github.com/login/oauth/access_token'
OAUTH2_AUTHORIZATION_URL = 'https://github.com/login/oauth/authorize'
OAUTH2_API_BASE_URL = 'https://api.github.com/'
OAUTH2_USERINFO_ENDPOINT = 'https://api.github.com/user'
OAUTH_ENDPOINT_NAME = 'user'