Add a HiDPI favicon.
minor #696 Add a HiDPI favicon. (whatthejeff)

This PR was merged into the 2.3 branch.

Discussion
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Add a HiDPI favicon.

Difference on my retina display:

![buvjv7biuaa5wwo](https://cloud.githubusercontent.com/assets/306525/3951439/8058ac42-26d6-11e4-89f4-3d69904f7b4c.png)

Commits
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c81a658 Add a HiDPI favicon.