Use Ember getter to retrieve the RESTAdapter headers.

The `host` and `namespace` properties on `DS.RESTAdapter` can be computed properties. However, `headers` somehow cannot. This change adds computed property support for `headers`.

In my case, my use case is setting the *Authorization* header, which is dynamic since a user can login / logout at any point in time. With this change, I can now simply do:

```
Ember.RESTAdapter.extend({
  ...
  headers: function() {
    return {
      Authorization    : myUserToken,
      X-Another-Header : 'Foo'
    };
  }.property('myUserToken'),
  ...
});
```