[BitmaskCollisionFilter]: improve with category and collision mask

[costashatz]

This PR fixes #136... The way to use this:

robot1:
    category_mask: 1 << 1
robot2:
    category_mask: 1 << 2
floor:
    category_mask: 1 << 3

This sets robot1 as category with number 2, robot2 as category with number 4, and floor as category with number 8. Now we set:

robot1:
    collision_mask: 4 | 8
robot2:
    collision_mask: 4
floor:
    collision_mask: 0

Collisions are ignored if collision_mask & category_mask_of_other == 0. In the above example, what will happen is that robot1 will collide with both robot2 and the floor, while robot2 will only collide with robot1.

Overall using the combinations of category and collision masks (per BodyNode), we can create a very complex collision ignorance systems.