Added support for Retina images

- Added a scss mixin for HiDPI background images.
- Updated CSS to use 2x images.
- Mapbox maps are serving 2x images without change (maybe new in our recent upgrade to 1.6.4).
- Map markers support 2x as well, but similarly to the map, will not dynamically change resolutions when moving a window to a Retina display.

Fixes #109. @pdehaan r?
I think autoprefixer will add the `-webkit-` prefix for us here.