Use NSLayoutAttribute instead of CGRectEdge values for geometry

[jspahrsummers]

This would lead to a more consistent and flexible API.

As part of this, all methods currently accepting a signal of edges should be changed to accept a constant NSLayoutAttribute, since 80% of the time there should be no reason to change the attribute that your layout occurs relative to (and the pain of boxing it in a signal isn't worth it for the 20%).