Keep the autocompletion list and entry field aligned.
Make index reset the responsibility of the autocompletion list.
CSS classes for the tagger and associated styles.
Mimic tag-it's menu structure.

Specifically, tag-it (and AFAIK all jQuery UI menus) represent menu
items as links.  Our existing styles are written to expect that, so
might as well play along.
Fix an off-by-one when pressing down to summon candidates.

(Yes, I know, tests.)
Keep selected candidate in view.

Scrolling code ripped from jQuery UI's Menu element.  I'm not yet sure
if the menu code can be reused wholesale.  (That'd be nice.)