Remove file extension for css classes

well, i figured that `.sprite-guy-png` is worse than `.sprite-guy`, so i changed this.

also your build system is sadly not very portable due to hardcoded paths. i ended up just concatenationg it via temporarily removing the dash from the appropriate target, then executing yuicompressor by hand. i think you’d get more mull requests if yourtoolchain would be easier to replicate.

thanks for the stitches, its spritesheet compressor is pretty awesome :)