add directive for uniqueness

[gad2103]

so that only one tag at a time can be added (no duplicates!)

[gad2103]

actually this is a bug when this is used with angular-boostrap, typeahead and BS3. i'm trying to figure out why, but the list doesn't filter correctly. its possible to keep adding the same tag over and over. it looks like the _filterSrcTag function is getting called but the changes to the tag array do not persist...