add a unique index to auth_user.username

Django 1.3 by default adds a unique index to the username column.
This is useful every time we do User.objects.get(username=u), as it
prevents us from doing a table scan.
pagination wrapper that takes an explicit list length
provide explicit number of times sounds and packs are downloaded

This may not exactly correspond with the number of sounds in the
sounds_download table (possibly related to #560) but it's within
about the right number for paging.
Adding migration file for index username
Merge pull request #577 from MTG/pagination

Provide explicit list lengths for pagination