Authenticated requests for non ember-data requests

Sometimes I need to make a JSON request to the server that does not involve an ember-data model. In these cases the call will not be automatically authenticated because logic for sending the auth token parameter exists in the Auth.RESTAdapter. 

This PR moves the logic into the Auth module itself so you can use it for non ember-data requests. 

For example, you can now make the following call from inside any controller:

```coffeescript
Auth.ajax('/api/non_standard_route', POST, {})
```