Bug 1016423 - Use Hawk for making authenticated HTTP requests and support FxA assertions

See bug https://bugzilla.mozilla.org/show_bug.cgi?id=1016423 please.
For the same session the credentials change (sign up with FxA and rest of calls with HAWK credentials or sign in with HAWK credentials) so a init() method based on the credentials the client uses makes no sense IMHO. Lets discuss this offline If you want.