@orta => added splitview

I added ORSplitStackView with a demo. I also added to the existing demos to highlight adding subviews to the middle of the stack view at a later point in time. Cleaned up some code from my last PR which was resulting in some warnings that for whatever reason were not being displayed before. Also fixed a crash for adding a topLayoutConstraint when the first view has a topMargin predicate of `nil` (which I think should be allowable).

As a side-note, I think that we _could_ get rid of the topLayoutGuide on ORStackView all together. Passing the ViewController's topLayoutGuide to an ORStackView results in a separate crash due to invalid constraints whenever the ORStackView in question is not the ViewController's `view`. If the stack view is some subview of the vc's `view`, the topLayoutGuide constraint made by ORStackView will be invalid. Personally, I feel like if a user wants to set up some constraints pertaining to the VC's topLayoutGuide, it should be up to them to do that as they will be more familiar with their views' hierarchy than we can possibly be. 

I'd still like to fix up the sideMargin bit some more as I've mentioned. Also worth noting is that `addSubviewWithTopMargin:whatever withSideMargin:nil` is different from `addSubviewWithTopMargin:whatever`. I'd like it if they were the same.