Switch to using Zopfli instead of plain gzip.

This switches assetgraph-builder to compress files using Zopfli (http://googledevelopers.blogspot.dk/2013/02/compress-data-more-densely-with-zopfli.html) instead of the standard gzip. Zopfli compressed files are on average 3-8% smaller than gzipped ones. As a downside, it take longer to compress, but I think this is an excellent trade-off for a tool like assetgraph.