Blender Git Loki

Added a new attribute type 'POINTER', for storing rigid body pointers.
This may not be the best way to map rigid bodies to particles,
eventually a nicer way could be to make use of the userpointer in Bullet
btRigidBody and store a weak reference to Blender types there instead.
Storing rigid body pointers in the particles just mimicks the way the
Object RBs work atm.

The pointer attribute gets a flag 'TEMPORARY' to prevent it from being
stored in the cache and .blend files, since it's only valid during the
Bullet stepping anyway.