Add ignoreDuplicates option with true, false, and notify options

Sometimes when loading a long list with AJAX, you actually want to have
duplicates from the list show up, in case the user forgets that they've
already added someone. This commit creates a new option, ignoreDuplicates,
with 3 settings:

true: Duplicates are ignored and not shown
false: Duplicates are added without any additional marking
'notify': Duplicates are added, but have options.duplicateNotification appended to their text

Note that the Prototype version does not seem to support duplicate filtering,
so this commit is jQuery only