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.
I was expecting the [three properties](https://github.com/vladikoff/fxa-docs-browser-loop/blob/master/docs/server.md#post-fxa-oauthtoken) from the OAuth server to be returned instead of just the token. Could we get all of the body from the /token request?