(WIP) FxA oauth support

https://bugzilla.mozilla.org/show_bug.cgi?id=1041814
Longer term, I think that legal would be happy if we weren't storing user's email addrs outside of FxA. The problem is that Loop wants for users to be able to contact each other via email address. If that's the case, we need a backend comm channel for Loop to ask for the uid associated with than email address, if it exists, likely from the FxA auth server.