Navigation bar and image loading improvements

I have created several improvements for styles:

### Loading of iPhone 5 specific images
Sometimes there's need to load iPhone 5 images (e.g. background image of view). The code I've attached allows doing this automatically by searching for images with -568h@2x suffix on iPhone 5 devices.

### Navigation bar in horizontal state
I've added background-image-landscape property to the Navigation Bar renderer, so view can have different navigation bar styles in landscape mode (this also includes support for setting custom image for iPhone 5 automatically)

### Allow specifying different states of navigation bar buttons
I've added several properties to the NUIBarButtonItemRenderer
* background-image-landscape
* background-image-highlighted-landscape
* background-image-disabled
* background-image-disabled-landscape

They are used only when background-image was specified.