OAuth2 (and some other random stuff)

Main stuff: 
Bunch of changes to Connection to better support oauth2, and addition of OAuthConnection for helping registration (fetching token, verifying signed payload) and making requests.
(changes to Connection has very little effect on its interface and behaviour, I think)

There's currently no OO style interface for OAuthConnection, though I'm fairly sure it'll be simple to add.

There's also currently absolutely no tests, since vcrpy doesn't like the project anymore (will try to use pymock instead, if vcrpy really doesn't work). My app is using OAuthConnection for all interactions with the v2 API, however, so there's at least some assurance that it works.