Implement collision groups and masks for bullet sim

[benbaarber]

Bullet has collision groups and masks for collision filtering which is a very useful feature, but gazebo does not support it through SDF. It would be great to add an sdf element under <collision><surface><contact><bullet><group/mask> or something similar to allow gazebo users to make use of this feature.

Somewhat related to #237. Rather than implementing it the way I described above, could pass <collide_bitmask> as mask and <category_bitmask> as group for bullet sim. Would be nice to solve #237 in the process so <category_bitmask> can be used with dartsim and tpe as well.

Side note, right now <category_bitmask> is defined and documented in the SDF spec here and there is no indication that this element actually does nothing at all (see my stack exchange post here). It would be nice if at the very least, if this feature isnt going to be implemented, to update the sdf spec to save people from rediscovering this and wasting time in the future.