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
Remove 'fuzzy' annotations from the messages file
Adds suggested postactivate script with some helper functions
Brings back the fix for document uploaded message translation

This fix was introduced in d170d5ce6 but was later lost in the merge process.
Marks tag labels in tagset for translation