Added Unique Value option to the Add Custom Meta and Edit Custom Meta screens. This allows you to create meta data which is exclusive to each user. If a user enters a duplicate value, the submission is terminated, an error message is returned, and the CSS class umm-error-field is added to the faulty field. Added total support for checkbox groups. Added new short code attributes. Added support for nested short codes. Updated Help section. Verified compatibility with latest WordPress 3.6.1.

git-svn-id: https://plugins.svn.wordpress.org/user-meta-manager/trunk@772900 b8457f37-d9ea-0310-8a92-e5e31aec5664