Add 2x2 large icons for retina display
Support retina media query for navigation icons

See
https://signalvnoise.com/posts/3271-easy-retina-ready-images-using-scss
for retina media query. Also `mask-size` makes retina images to work as
mask. See http://stackoverflow.com/a/16550763/3108885 for this.
Merge pull request #42 from yous/patch-large-icons

Add 2x2 large icons for retina display