add basic csv support

Hi there,

I was very glad to find a Tiled package on hackage for some project I'm working on recently. It words wonders :)

Though, I needed to parse some data blocks that use CSV so I took the liberty to add some (very basic) support for that. Feel free to merge and upload to hackage if it seems sensible :)

Sorry for the import whitespace mess, my emacs has a hook for that and I only saw that once commited.