WIP: Physics

Magical physics UI support for Ionic. This is a WIP branch.

- [x] Support subset collisions for bodies (like edge and gravity already do). Don't collide bodies that are not in the same item group
- [ ] Build out other behaviors
- [x] Evaluate "group naming" system for bodies defined by directives
- [ ] API Review
- [ ] Figure out what to do with PhysicsJS dependency, including the changes I made to it
- [x] Fix PhysicsJS's edge collision sticking bug
- [ ] Move touch handling, move to combination of user supplied gesture and behavior (push, attach, etc)
- [ ] More Tests