Adds horizontal layout support to SC.GridView

Here is my initial work to add horizontal layout direction support to SC.GridView. There is still some work to do (unit tests mainly)...

I'm publishing it because there is some stuff to discuss. I needed horizontal support to make the same kind of view that you can see on the iOS home screen. For this, we need a notion of page. I already added two properties about that `itemsPerPage` and `pageCount` but, I now think that we should create a new kind of view which will extend `SC.GridView`. This view could be call `SC.SlideView` or `SC.GridPageView` or something else. 
I think we need a new kind of view because items should be layout per row and not by column like it is the case in the current implement when `layoutDirection` is set to `SC.LAYOUT_HORIZONTAL`. We will also need public methods like `pageIndex`, `showPreviousPage`, `showNextPage`...

So first, I'd like to know if you're agree with me that we need a new kind of view. And if so, we need to agree on a name.