Add a 2x Glyphicons sprite sheet

Summary:
Add a 2x ("retina") sprite sheet with icons that I gave some hover/active effects. I'm just doing one sheet rather than separate 1x and 2x sheets, we can muck with it later but I don't think anyone's going to go over their bandwidth cap.

@chad, I'll put the PSD on the Dropbox too if you have a chance to give it a once-over.

Test Plan: Built menu on this, all the icons work.

Reviewers: btrahan, vrana, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T1569

Differential Revision: https://secure.phabricator.com/D3102