Physics scroll view demo

This is a demo I helped work on back in July (sorry for the huge delay in upstreaming). It's an extension on scrollview, and adds some simple physics-like effects to the scroller.

I've been meaning to clean it up for a long time, but things keep changing upstream, so I decided it might be better to just land now with a few TODOs and clean it up bit by bit from here on.

I'm sure a lot of this isn't in FOAM best practices, suggestions and criticisms welcome.