Added group and mask properties to Rigidbody component.

Group defines which collision group the body belongs to, each type has a default (DEFAULT, STATIC, KINEMATIC, TRIGGER; for dynamic, static, kinematic, triggers)
Mask defines with which other groups collisions occur. Each type has a default (ALL, NOT_STATIC_KINEMATIC, NOT_STATIC, NOT_STATIC; for dynamic, static, kinematic, triggers)

New properties available on rigidbody: entity.rigidbody.group, entity.rigidbody.mask
Updated all constants to be in pc.* namespace
Moved all constants in to rigidbody_constants.js
Update constants to match existing definitions in Bullet
Change event.fire() so that it checks for callbacks before doing allocations.
Merge pull request #79 from playcanvas/feature/collision-layers

Added group and mask properties to RigidBody component.