Use unprocessed contents of files for css hashes

Suggested by justinrandell here: http://drupal.org/node/1014086#comment-3974410

This is already the case for JavaScript hashes since we don't do any processing anyway. Profiled this with xdebug and it reduced the time spent generating the filenames in a stock core install from 25ms to 2ms. With contributed modules enabled there'll be more css to process, so the savings should be greater.

About to commit this patch to HEAD.