release version 0.6.4


git-svn-id: https://plugins.svn.wordpress.org/brutal-cache/trunk@783833 b8457f37-d9ea-0310-8a92-e5e31aec5664
updated version number in files


git-svn-id: https://plugins.svn.wordpress.org/brutal-cache/trunk@783843 b8457f37-d9ea-0310-8a92-e5e31aec5664
added is_string


git-svn-id: https://plugins.svn.wordpress.org/brutal-cache/trunk@784744 b8457f37-d9ea-0310-8a92-e5e31aec5664