Add South

- Add it to requirements and Django project
- Create the first (fake) migration

Note that it probably needs some care when deploying for the first time with
South.
Add Whoosh to requirements
Add whoosh's schema to core/models
Add a helper WhooshIndex class

This class is just a little layer on top of whoosh and the idea is to
facilitate the use of it (even indexing or searching)
Add `indexed` field (default=False) to `Document`
Add 'indexed' (yes/no) to document metadata
Add management command 'update_index'

This command search for documents that are not indexed and for each of them,
verify if the 'text' property exists. If it exists, it adds the document to the
index.
There is a weird exception raising at the end of running 'manage update_index'
related to GridFSStorage but as we are not using it in this command and all
operations are executed correctly, I'm ignoring it.
Add copyright information to 'manage update_index'
Merge branch 'develop' into feature/full-text-search
Fix a typo (syntax error) in visualizations
Add full-text-search test boilerplate
Add test for manage.py update_index
Add --verbose to update_index (quiet = default)
[FTS] setUp/tearDown: delete index and database
Add basic search test and view
[FTS] Show only documents owned by user
Change 'content' to be stored

If we store a field using Whoosh, we can then use the 'highlights'
(concordance) when searching.
Generalize filtering by owner + store concordance

- The function that searches for documents and filter by owner is outside view
  function
- Store an attribute 'concordance' in each returned document
Add template for search page
Add search filtering by corpus