Bugfix/mgorbach/autolayout common ancestor

The `commonAncestorForViews:` method should be useful in the future as we add more interesting things to this category.

@Raizlabs/maintainers-ios: please review.
This PR comes from an issue I found: the `rz_pinHeightToView` and `rz_pinWidthToView` methods worked correctly only if the passed-in view was a subview of `self`. 

This is an unnecessary limitation. I've made them significantly more  flexible by finding the common ancestor and applying the constraint in the right place.
Likely, but there would still be some work to do. You would still have to traverse. Maybe you could go up the chain of one view and ask each view whether the other view `isDescendentOfView`? The issue is that wouldn't scale to > 2 views.