Preliminary Gameplay Prototype (step 3)

A third iteration of the eater gameplay prototype. Again, to be implemented in all the candidate frameworks listed here: https://blueprints.launchpad.net/eaterworld/+spec/client-framework-choice . At this point we can start ranking the different frameworks in terms of ease of change; but it's probably too early to start eliminating.

New Gameplay Elements
* Eater NPCs (slow, random motion---as a proto, this version should be almost trivial to write and to play)
* When eater A's head collides with eater B's body, eater A eats a chunk off eater B's mass. The eat rate depends on the size of A's head.

Alternatives to test
* Can eaters die when they lose all their mass?
* Winning criteria: reach a certain mass, kill all other eaters.