Resource definitions should be able to proxy DS methods

How it works now:

``` js
DS.defineResource('user');

DS.find('user', 5);
DS.findAll('user', {...});
// etc
```

How it will work:

``` js
var User = DS.defineResource('user');

User.find(5); 
DS.find('user', 5); // works same as above

User.findAll({...});
DS.findAll('user', {...}); // works same as above

// etc
```
