Make TiledLayer strip leading whitespace from csvData

FlxTilemap blows up if the first character in a map CSV is a newline. On
at least the OS X 0.10.2 release of Tiled, layers are stored in .tmx
files with a leading newline before the actual CSV data. In an effort to
make FlxTilemap not blow up, let's just strip any leading whitespace
when we get csvData from TiledLayer.
Merge pull request #126 from killpack/dev

Make TiledLayer strip leading whitespace from csvData