Allow pinning edges of views that don't share the same immediate superview.

I added another variant of edge pinning: `pinEdge:toEdge:ofView:withSuperview:inset:`. This allows you to pin the edges of views, but passing in a specific superview to apply the constraints to.

`pinEdge:toEdge:ofView:inset:` now calls this new method instead, and I've also updated the `superview == peerView.superview` assert message to recommend use of the new alternative method.

Does this look okay to you?