fixes #17: set Auth.currentUser to null on signout
changelog update
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
changelog; version bump
Config param to enable header auth.
Use headers to pass auth token if requestHeaderAuthorization is true
The key for the request header is now configurable.
Updated readme.
Merge pull request #19 from seanrucker/request-header-authentication

Using request headers for passing the auth token