Discourage the use of Em.A

Em.A isn't redundant and just makes our code more verbose without adding any functionality. From [Ember.A](http://emberjs.com/api/classes/Ember.html#method_A)

```
Creates an Ember.NativeArray from an Array like object. Does not modify the original object. Ember.A is not needed if Ember.EXTEND_PROTOTYPES is true (the default value). However, it is recommended that you use Ember.A when creating addons for ember or when you can not guarantee that Ember.EXTEND_PROTOTYPES will be true.
```

Unless we're not sure something is an array (e.g. function arguments) I think we should avoid it. 

Unless there're some issues, I think it's better to deprecate the use. Extending the prototype makes this two objects the same. See as a reference:

![EmA](https://s3.amazonaws.com/uploads.hipchat.com/22555/778938/Cxto4Qy7c52fQhS/upload.png)