Implemented local collision checks

* Methods in TileUtil return a list of entities that are likely to
collide/overlap with a given position or Tile Location
* Added lists of actors and effects to the GameWorld class, added in
addEntity
Added unit tests.
Fixed bugs

* removed mapTiles[][] initialization in GameWorld constructor -- should
be null unless the parser is used or the tiles are set manually. This
was causing the errors in TileUtil.
* Changed unit tests for TileUtil to use both overloaded and
non-overloaded methods.
* Added getClosestTileX/Y to TileUtil.
Removed print statement
Merge pull request #303 from BU-CS673/Aaron

Implemented local collision checks for Tank