feat(retina) Directive ng-src supports Retina displays

A port of the Javascript library http://retinajs.com/ for AngularJS with the possibility to use angular markups.
Elements containing ``<img ng-src="...">`` now can handle high resolution images, if the browser runs on a high resolution (Retina) display. Otherwise, or, if the hires image can't be found, the default image is loaded.
This module is very useful for everybody optimizing their application for Retina display such as the newer iPhone's, iPad's and MacBookPro's.

This is a rebased version of #2032