Numerous fixes and additions.

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.