Distribute-by-rupture phase 2

The event based calculator is now able to compute the GMFs from the ruptures stored in the database,
see https://bugs.launchpad.net/oq-engine/+bug/1321681

NB: the companion PR https://github.com/gem/oq-commonlib/pull/24 is needed.
Please rename all_ruptures and tot_ruptures to make their meaning clearer