Make pinning the width or height of one view to another work when both v...

...iews are peers, or even if they have any 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.
This might be more performant (and concise) if you convert all the hierarchy arrays to `NSOrderedSet` and intersect them all. The first common ancestor should be the first object in the result.