Add retina versions of all our icons.

Review Request #4362 — Created July 28, 2013 and submitted — Latest diff uploaded

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Add retina versions of all our icons.

I've redone all the icons as SVGs and fixed up parts of the icons that I
thought could be improved. With this SVG master, we can now generate 2x
icons for Retina displays.

The .rb-icon rule has been updated to select icons from the new
spritesheet when on a Retina display.
Tested each icon on both non-Retina displays and Retina displays, in Chrome
and Firefox.