Bug 1041814 — FxA oauth support

Integration with Firefox Accounts using OAuth.
https://bugzilla.mozilla.org/show_bug.cgi?id=1041814

- [x] Implementation of the OAuth endpoints
- [x] tests
- [x] Update the changelog accordingly
- [ ] Test that it really works when configured properly.
[This](https://github.com/vladikoff/fxa-docs-browser-loop/blob/master/docs/server.md#endpoints) is what I was following for the Firefox implementation (some of which has landed). 
I'm passing the state and code in the body of the request, not as GET query parameters.