allow UIViewController subclasses to be used in stylesheet

- make CASStyleableItem consistent with UIKit
- add cas_alternativeParent to allow CASStyleableItem to have up to two parents
- UIViewController implements CASStyleableItem
- override UIViewController setView: to assign UIViewController as view's cas_alternativeParent
unit tests
Merge pull request #8 from cloudkite/uiviewcontroller-parent

Ability to specify UIViewController class/subclass in style selector hierarchy