Implement bulk insert using Django

[Mattia Barbon]

The current bulk insert implementation uses SQLAlchemy; it should be rewritten to use Django ORM.

Depends from https://bugs.launchpad.net/bugs/820346
Related to https://bugs.launchpad.net/bugs/809780

There isn't a bulk inserter built-in in Django; some third party solutions:

http://djangosnippets.org/snippets/446/
+ very complete and handles relations (not needed for now but nice)
- not maintained (last updated in 2008)
- might be a pain to debug/update

http://djangosnippets.org/snippets/2362/
+ simple and basically what we have now
- does not handle relations (but we do not need them ATM)

http://people.iola.dk/olau/python/bulkops.py
Similar to snippet 2362 but uses .executemany rather than bulk insert
- executemany is slower than bulk insert

http://stackoverflow.com/questions/2655670/how-do-i-perform-a-batch-insert-in-django
Uses a custom manager so the bulk insert method is available directly in the model class
- I like the separate bulk insert class better, but it's a personal preference

[Mattia Barbon (mattia.barbon)]

Pull request https://github.com/gem/openquake/pull/440