Implemented losses per event per asset

See https://bugs.launchpad.net/oq-engine/+bug/1383291 and the companion PR https://github.com/gem/oq-commonlib/pull/45. The tests are running here: https://ci.openquake.org/job/zdevel_oq-engine/766
No, they are all NOT NULL (see the SQL table definition). I will update the Django model.