Clarify key handling code

Cascades with postfix conditionals read easier than a chain of if-elses.
Add box2dweb library

http://code.google.com/p/box2dweb/

It's a port of Box2DFlash automatically converted to JavaScript.
Make box2dweb `require`-able

Pretty trivial.
Port physics, collisions and player input to Box2D

The `Space` scene now instantiates a Box2D physics world that the
entities modify as they're created. [The Box2D Manual][1] may be
helpful when reading, as the API of the Flash port (and consequently the
JavaScript port used here) closely mirrors the original C++ Box2D's API.

 [1]: http://www.box2d.org/manual.html "Box2D v2.2.0 User Manual"
Port planet absorption to Box2D

Centralise object destruction to the EntityManager. Planets' fates are
never decided at their internal scope, so it makes sense for the manager
(which has a more global view) to control their creation and
destruction.

Got rid of `utils.ls`' `List` while I was at it. Just plain Arrays can
handle it.
Port gravitation between planets to Box2D

Also gave initial planets an initial "spin" to get them started on an
interesting trajectory.
Add `README.md`

This GIF demo explains it a lot better than I can.
Merge pull request #1 from Paraknight/box2d-port

Port physics to Box2D