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.
If the views have no common ancestor, might it be useful to throw a warning/error/exception here? Might be easier to interpret than the default exception that the system throws.