Add more grp support for getTerrainContactPoints.

Now you can pass a single cell-array of strings to use the same groups for all
bodies.
Don't check terrain collisions unnecessarily.

If specific groups are specified in `active_collision_options` and `terrain` is
not one of them, don't check terrain collisions.
Use active groups when checking terrain collisions

Pass the groups specified by `active_collision_options` into
`getTerrainContactPoints`
Update testActiveCollisionOptions.

Now we check both bullet (if applicable) and non-bullet (i.e. terrain_only)
collision detection.

Resolves #364.
Tweak terrain collision detect logic

Running `collisionDetect` with `active_collision_options.terrain_only` set to
`true` now implies that you want to check for collisions against the `terrain`
group. Or rather, it always implied that, but now the code actually respects
that.
Remove no-longer-necessary hack from QPController.

Takes out a bit of code that was needed because `collisionDetect` didn't
respect filtering by group for terrain collisions.
Merge pull request #447 from avalenzu/fix-testActiveCollisionOptions

Fix testActiveCollisionOptions