Added username sanitization, minor code cleanup

git-svn-id: https://plugins.svn.wordpress.org/username-changer/trunk@862641 b8457f37-d9ea-0310-8a92-e5e31aec5664
Added multisite support

git-svn-id: https://plugins.svn.wordpress.org/username-changer/trunk@862701 b8457f37-d9ea-0310-8a92-e5e31aec5664