Updated favicon.ico to include retina assets and migrated from GL to cat glyph

The new favicon displays the GitLab cat instead of "GL", and packs 64x64px, 32x32px, 24x24px and 16x16px assets. Recent browsers on retina displays will render the 32x32px asset. Older browsers automatically fall back to the regular 16x16px asset. The 64x64px asset is included for future-proofing, and the 24x24px asset for coverage.

Unlike #4680, adding retina favicons in this manner is fairly standard.

![logo-black](https://f.cloud.github.com/assets/1771288/1642536/601678d0-588a-11e3-9ec5-04c6d1826b6d.png)