Rough sketch of the `list calculations` views

The first major piece of functionality to address the following:
https://bugs.launchpad.net/oq-engine/+bug/1201317

At the moment, there is no authentication; this still needs to be added.
If you have a working oq-engine installation, you can easily test these views by doing

`python manage.py runserver`

and then hitting 127.0.0.1:8000/calc/hazard or /calc/risk.