[#103] Improvements in collision detection.

- Fixes enough to make the Platformer playable and the smilies in collision_test stack nicely!
- Known bugs remaining:
  * Some smilies will still "group" together and fall slower than the rest. This is caused by callbacks being run twice on colliding pairs (see FIXME in me.collision.check()) -- Also a performance issue
  * Jumping in Platformer against a solid wall of tiles will cause a false-positive that kills vertical velocity when the corner of the hitbox touches the bottom of a tile (see FIXME in me.collision.calcResponse())
  * It's not possible to fully prevent all collision responses (see FIXME in me.collision.calcResponse())
- Only "solid" tile collision response is implemented. (TODO: slopes, ladder, platform, ...)