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

The overlapping check performed for tiles with allowCollision = false ignored the object widht/height, so the tile callback would fire even when the Object was placed close to, but not overlapping, the tile.
Merge pull request #166 from Dovyski/fix_issue_37

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