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.
IMHO we should keep these two functions.

> I don't see why we have 2 functions. Everytime a new user is logged, we need to set again the
> 'hawkCredentials' and store them in our AccountHelper

Nope, when the user signs up uses BrowserId or MSISDN-based authorization headers or no authorization header. Moreover we derive the HAWK credentials the client will use for the rest of request and store the account. We have to do this every time a new user signs up.

When the user sign in uses HAWK authorization header and we do nothing else.