Move copyright/license attribution to license.txt. Add reference to famfamfam Flag icons.

Following http://make.wordpress.org/themes/2014/07/08/proper-copyrightlicense-attribution-for-themes/

git-svn-id: https://plugins.svn.wordpress.org/bogo/trunk@959527 b8457f37-d9ea-0310-8a92-e5e31aec5664