Feature/full text search

These commits implements full-text search using whoosh. Note that:

- It's needed to store the index somewhere in the filesystem, so we need to defnie `INDEX_PATH` in `settings`
- I've added south to do migrations in the database
- It's needed to run `manage.py update_index --settings=settings.xxx` to update the index with new uploaded documents. This script verifies if there is another instance of it running (if yes, do nothing; if no, index not-indexed documents) so we can add it easily to crontab every minute (we need to create the code to add this script to crontab on deployment - https://github.com/NAMD/pypln-deploy/issues/6).