Added Ember.computed.flattenArray

Ember.computed.flattenArray proposal to allow computed flattenning of arrays. E.g.:

```javascript
var item1 = Ember.Object.create({
  tags: [ 'important', 'bug', 'task' ],
});

var item2 = Ember.Object.create({
  tags: [ 'urgent', 'feature', 'task' ],
});

App.ItemList = Ember.ArrayProxy.extend({
  allTags: Ember.computed.flattenArray( 'content', 'tags' )
});

var myList = App.ItemList.create({ content: [ item1, item2 ] });
myList.get( 'allTags' );
// -> ["important", "bug", "task", "urgent", "feature", "task"]
```