Fix issue #37 FlxTilemap.overlapsWithCallback calls callback when no overlapping

The overlapping check performed for tiles with ```allowCollision = false``` ignored the object width/height, so the tile callback would fire even when the Object was placed close to, but not overlapping, the tile.

Fix: 
https://github.com/FlixelCommunity/flixel/issues/37
https://github.com/AdamAtomic/flixel/issues/194