Add visual design guidelines for Firefox OS
Bugs 870998 872747 8873482 872747 884424 863160
Merge pull request #1034 from sgarrity/bug-870998-firefoxos-type

Add visual design guidelines for Firefox OS
at2x retina image function - refs bug 898013

Mozilla.ImageHelper class that handles detecting high-res display and also handles showing high-res images as well as handling platform images
Merge pull request #1119 from nburka/bug-898013-retina-img-tag

at2x retina image function - fix bug 898013