Reset widthInTiles on multiple loadMap calls

Fixes the issue in FlxTilemap.loadMap where calling 
it on the same FlxTilemap more than once
wouldn't reset the widthInTiles field.
Merge pull request #281 from SeanHogan/flixelNME

Reset widthInTiles on multiple loadMap calls
Merge remote-tracking branch 'origin/flixelNME' into dev