Ember-model custom adapter support

I implemented a config option which allows users to put a specified `RESTAdapter` in place of the default `Ember.RESTAdapter`. An example config looks like this:

```coffeescript
App.Auth = Ember.Auth.create
  # ...
  emberModel:
    adapter: 'App.RESTAdapter'
```

I've attached a test case which checks if the emberModel.adapter option defaults to `Ember.RESTAdapter' so that this patch doesn't break backward compatibility, but I'm not sure what's a good way to test if a custom adapter is actually working.

P.S. I guess we can do the same thing in the ember-data module.