Improve Phabricator favicon

Summary:
See http://www.facebook.com/epriestley/posts/923755264939. The favicon is too
complicated and not distinct. Use a quarter-slice of the gear instead and also I
resized it better in Photoshop.

Test Plan:
I'm a faaaaaabulous designer!

Reviewed By: aran
Reviewers: rayhe, tuomaspelkonen, jungejason, aran, tomo
CC: aran, epriestley
Differential Revision: 455