Added support for a link item to reside in multiple groups.

[thedudeguy]

for example a "Home" link, can be be in group "Header Nav" and group "Footer Nav". or "Header - Logged in" and "Header - Logged Out" simultaneously. The select form in the admin was changed to a multiselect. The helper to show the nav menu uses some persistent data so that it doesn't have to make a lot of db queries.

Made sure to add backwards compatability so that existing links set on a nav shouldn't get broken when upgrading the module.