atname quickselect, powered by jquery-ui autocomplete

[thehunmonkgroup]

This pops up an autocomplete widget any time someone begins
typing an atname. A user's name can be selected by clicking,
or navigating with the arrow keys and hitting either enter
or tab.

The typing user's name is excluded from the list.

[thehunmonkgroup]

Couple of additional comments about this PR:

• the selectionStart element property being used in the source: function doesn't work in IE, apparently: http://stackoverflow.com/questions/2628323/textarea-selectionstart-in-ie -- i tested in FF, Chrome, Opera, and Safari and it works perfectly, and i have no way to test on IE. so some workaround may need to be found there.
• there is an existing bug in the atname code, where two users with the same first name may not get the correct highlighting. i believe the correct way to solve this is to use the user's full name for atnames, substituting spaces with underscores. this would basically create unique chat nicknames and solve this problem. regardless, this patch merely exposes the bug.

[yourcelf]

there is an existing bug in the atname code

Omitting spaces should work with the existing code. So "Charlie DeTar" maps to "@CharlieDeTar", case insensitive. I think that tracks to the loose conventions people use with e.g. twitter names, so I'd rather keep it there than use underscores. See atname.normalize for the existing name normalization (not much to it).

This looks cool -- my main concern is just the heavy-weight-ness of loading in a whole new UI framework (jquery-UI) that has different style conventions just for the one feature. Bootstrap2 used to have a built-in typeahhttps://github.com/bassjobsen/Bootstrap-3-Typeaheadead plugin... it looks like there have been some efforts to port that to bootstrap 3, though I haven't evaluated that either yet. There's also twitter's typeahead plugin, but that might be a little overkill for this too.