Include HiDPI version of favicon.

Include the latest version of the favicon from HTML5 Boilerplate. This now supports HiDPI displays. The favicon itself has two resolutions: 16x16px and 32x32px.

Please also see h5bp/html5-boilerplate#1286 and h5bp/html5-boilerplate@21702c4198124647d541be635268b16ef7d33c72.