Include Select2 and a basic implementation in wp_dropdown_users().

Queries users in a dumb fashion at the moment (no preference for
users that write a lot), but does scale with search and basic
pagination.
Make sure the selected user's name shows up as selected, fix infinite scroll.
@todo
An autocomplete field should only show if the `autocomplete` flag
is set in the function invocation's options hash, or when in the
admin more than 100 users are returned for the query.

Introduce wp.select2, which stores query arguments per dropdown field
instance, which is passed to the user query AJAX call.

Fix some documentation, use boolean defaults rather than `0` or `1`.

First run at l10n.

Allow numerous instances of an autocomplete dropdown.
Remove unnecessary select2 files
Merge pull request #2 from helenhousandi/select2-19867-dev

Include Select2 and a basic implementation in wp_dropdown_users().