Implemented disabling and enabling collisions

Collisions will now only be checked if
PhysicsBody.canCollideWith(EntityType type) returns true. By default,
all PhysicsBodies may collide with all other PhysicsBodies. Collisions
can be set by PhysicsBody.setCollidableWith(EntityType, flag).
Merge branch 'collision-toggling' of https://github.com/ArchieMonji/MergeToSurvive into collision-toggling