Collisions, Tiling, Slippery Walls

* Added Polygon-based bounding/collisions to Entities
* Added 'solid' parameter to Entities, for determining whether mobile
objects should stop movement on collision
* Modified Tank movement to respond appropriately to frontal collisions
(sliding)
* Unsure how to unit test the getOverlappingEntities and
getLookaheadEntities methods! They're both Protected/Private, so they
won't break any other systems, but this could be improved.
* Added CollisionTestApplication. Drive around and try not to run into
Pillboxes!