Add a chunk method to Collection

Let's add a `chunk` method to `Collection`. We already implement most of php's array methods, so why not.

People ask about this a lot in IRC, since it's very handy when building a grid out of items:

```html
@foreach( $products->chunk(3) as $chunk )
	<div class="row">
		@foreach( $chunk as $product )
			<div class="col-xs-4">{{ $product->name }}</div>
		@endforeach
	</div>
@endforeach
```

Note that `$chunk`s themselves are collections :)