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
- [ ] Update the documentation to reflect these new APIs
- [ ] Update the changelog accordingly
- [ ] Test that it really works when configured properly.
It looks like `POST /fxa-oauth/token` returns it, isn't it enough?