Prepare the Sprite plug-in for collision handling

With the current design the objects that can be involved in a collision
should expose a addCollisionHandler() method so that controllers can
be notified of collisions, and the object is also responsible for
calling these when it's notified about collisions.
Handle collision in SimpleController

Add some basic implementation for "handling" collisions in the
SimpleController plug-in. Currently it's broken because it doesn't
handle collisions correctly, it just negates the speed vectors.

It also doesn't do any real resolution of collisions currently, which it
has to do.
Add initialize method for KeyboardController
Add basic collision manager

The collision manager is responsible for keeping track of objects that
should be checked for collisions and notifying objects when that
happens.
Add collision manager as a required resource
Incorporate CollisionManager in the Game object
Merge pull request #11 from RobertN/collision-handling

Collision handling