fixed bug in quadtree.retrieve

returned array of potential colliders no longer contains duplicates
Quadtrees now process Stage's children. +  Swarm refactoring

The QuadTree structure checks each child of the Stage to see if it's a
Destructible. If yes, it gets added to the quadtree.

* Removed the "Game.colliders" array, as this purpose is now served by
the Stage.children array.
* Modified related classes to fit the changes.
Merge branch 'master' of https://github.com/Stopa/Mobile-Defence-Fortress

Conflicts:
	assets/javascripts/actor/Enemy.js
	assets/javascripts/collision/Collision.js