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.
Merge branch 'master' of github.com:imulus/retinajs into fixes

Conflicts:
	src/retina.js
Makes dependencies be installed locally.