Add support for gzip compressed TMX data

I've had this sitting around on a branch for a while, and figured I'd try to pull it in.  The old behavior of uncompress() did not work with gzip data.  I updated the method to pretty much imitate the way that cocos2d's TMX loader handles compressed data, and this version should work with both zlib and gzip.

EDIT: Apologies for the messiness of this branch.