Auto-generate cache fingerprints of images during packaging

Modify the CSS compressor to automatically add a fingerprint to all image files. The fingerprint is based on the checksum of the file so it'll only change when the contents of the file have changed.


    .myelement { background:url('/css/pph2/sprites/common.png') no-repeat 0 0;
    --->
    .myelement { background:url('/css/pph2/sprites/common.png?80bbf293') no-repeat 0 0;