Supports data-at2x attributes.

Uses supplied data-at2x attributes for the @2x asset URLs instead of doing HEAD requests.
Added unit and functional tests for data-at2x attributes support
Added data-at2x support and hints about Rails usage to README.
Clarified asset digest a bit in README.
Clean up README and move license into LICENSE file
Beef up the Makefile
Version bump
Remove extra quotes around background-image url
Merge in master