Moved the logic responsible for adding the auth token to ajax requests.

Moving it out of the adapter and into the Auth class allows us to use it for requests that are outside of ember-data but still need to be authenticated.

E.g. We could call Auth.ajax('/api/non_standard_route', POST, {…}) from within a controller.
Updated readme to explain new way of making authenticated requests.
Merge pull request #18 from seanrucker/authenticated-requests