App Engine unfortunately can't make outbound requests without using
the appengine urlfetch library and a request-specific context.

I don't see a way around this other than to change the signature of
the Provider and Session interfaces.

This really has a huge amount of downsides obviously, but before I
go and fix stuff for providers besides Facebook, G+, and Twitter
(Twitter support requires mrjones/oauth#60
to get merged), I figured I'd open a pull request and discuss the
tradeoffs.

Unfortunately I think we need to do something like this to support
App Engine. Perhaps this is an opportunity to version goth using
gopkg.in or something?