Haystack indexes

Haystack indexes are static (they use the indexes.SearchIndex class), they are reindexed with the help of manage.py command.
We could use indexes.RealTimeSearchIndex to avoid nightly reindexing process.

This is not a so easy change. Because, index fields are not django field but rdf triples. Thus it is impossible to use indexes.RealTimeSearchIndex when rdf objects are created (exception is raised because triples are not in rdf bd when the djrdf object is created).