Oauth workflows

Thought I'd migrate the bulk of the current README to an OAuth1 workflow section of the upcoming docs (#39) to make it more clear/inviting to new comers. Also added a parallel track to the docs section showing how to use OAuth1Session.

Threw together a quick guide showing OAuth2 WebApplication flow with OAuth2Session as well. 

Obs! The README changes add links to requests-oauthlib.readthedocs.org which is not yet setup...