Bullet changes, to test collision between objects via API

Squashed commit of the following:

commit 6eabff94f1daf726c8fbc50f6d2690446ee54dda
Author: Benoit Bolsee <benoit.bolsee@online.be>
Date:   Mon Dec 30 12:12:46 2013 +0100

    bug fix: ignore self object in convex_sweep_test.

commit ad2185abff85d4b8f480b8a569444bac465a202e
Author: Benoit Bolsee <benoit.bolsee@online.be>
Date:   Sun Dec 29 11:56:24 2013 +0100

    new RB_dworld_get_collision_paris() to report the collisions that occurred
    in last frame.

commit 6e794c3deab6be57c283021cd193eba2413200e5
Author: Benoit Bolsee <benoit.bolsee@online.be>
Date:   Fri Dec 27 17:46:31 2013 +0100

    Add 2 param to rigidbody_world.convex_sweep_test:
    col_groups to indicate which groups we want to test
    use_sensor to indicate if we want to detect sensor objects (default no)
    return is a 5-uple, last value=reference to object hit (only if 4th
    value=1)

commit 4c6b21beeeb24225591e939878faa97c61a99afb
Author: Benoit Bolsee <benoit.bolsee@online.be>
Date:   Thu Dec 26 13:49:38 2013 +0100

    Add sensor physics type.
    A sensor can collision with any other objects, including other sensors
    but does not restitue any force. Collision is controlled by a new
    setting: collision mask. A sensor collides with another object
    if the object group is part of the its mask.