add favicon support

Updated pull request to base on master instead of rails 4 branch. 
Original issue: https://github.com/gregbell/active_admin/issues/747
Pull request that was never resolved and additional discussion: https://github.com/gregbell/active_admin/pull/979
Closed pull request (based on rails4 branch): https://github.com/gregbell/active_admin/pull/2347, closed in favor of this one