Add group bitmask filtering for PhysicsCollisionObject

PhysicsCharacter, PhysicsGhostObject and PhysicsRigidBody now supports
collision filtering using bitmasks.

See issue #1029 on github.
Changed shorts to ints for collision group bitfields.
Merge pull request #1053 from rgngl/physicsFilters

Add group bitmask filtering for PhysicsCollisionObject