Fixes #50 by adding a description field (see WARNING below)

Adds a description field that can be added to each checklist and..

- Shows a truncated description in the search results
- Shows a trunctated (but expandable) description on the view view.
- Does not yet search description (but should when previous PR is merged)
- Adds south - see below...

WARNING: This PR introduces south.

You will need to syncdb to get the south tables added, but DO NOT RUN MIGRATE.
There are some issues:

1. You may well have to cheat if you have an earlier DB by not running the initial
   migration.

   insert into south_migrationhistory values(1, "main", "0001_initial", date());

2. If already setup you may need to fake the taggit migrations as follows:

   ./manage.py migrate taggit --fake