Include HardonCollider (http://vrld.github.io/HardonCollider/) as a submodule
Rewrite collision code to use the HardonCollider library

Notable changes:
  - Entity:checkCollision has been removed
  - Moving an entity to a new position *must* be done with Entity:move
  if you want the hitbox to follow.
SPlit out collision code to separate physics module
Add object destructors, where the Entity destructor removes the hitboxes
Add point hitboxes for explosions