Hide the image initially to prevent ugly squares 
Hide the image initially to prevent ugly squares 

This prevents the ugly squares that appear while ng-retina is performing the initial OPTIONS request.

The image is simply made invisible until ng-retina has determined which URL to use, and calls `setImgSrc` to set it.
jrief/angular-retina,joujiahe/angular-retina,joujiahe/angular-retina,jrief/angular-retina