Type parameter of registerAdapter method can be now a string or an array of string

Instead of
```javascript
App.Store.registerAdapter('App.User', App.myRESTAdapter);
App.Store.registerAdapter('App.Post', App.myRESTAdapter);
App.Store.registerAdapter('App.Comment', App.myRESTAdapter);
```
now you can do
```javascript
App.Store.registerAdapter(['App.User', 'App.Post', 'App.Comment'], App.myRESTAdapter);
```