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.