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.
Take into account you have an `onerror` which is never used. We could follow 2 approach here:
- onsuccess/onerror approach
- onsuccess(error, result)

In this case this line should be or `_callback(onerror, [null]);` or `_callback(onsuccess, ['There is no account']);`, changint as well line 35.