Implemented Collision toggling

Created branch to test collision toggling. Collisions can be enabled or
disabled by calling PhysicsBody.setColliableWith(EntityType,boolean).
Implemented with bitmasking for now. Added Items and Projectiles as
EntityTypes.
Merge branch 'collision-toggling' of https://github.com/ArchieMonji/MergeToSurvive into collision-toggling