Remove 'time span' from stochastic event set and disaggregation calculators

Fix https://bugs.launchpad.net/oq-hazardlib/+bug/1276830

It should be noticed that, in the current master, the stochastic event set calculator is broken because the rupture iterator still takes as parameter the temporal occurrence model (see https://github.com/gem/oq-hazardlib/blob/master/openquake/hazardlib/calc/stochastic.py#L62) while it has been removed in https://github.com/gem/oq-hazardlib/pull/142. However no test is broken.

In this pull request I also added a new acceptance test for the stochastic event set calculator that reveals the problem.

``stochastic_event_set_poissonian`` has been renamed to ``stochastic_event_set`` and ``disaggregation_poissonian`` has been renamed ``disaggregation``. This last change requires also a change in oq-engine (that call the disaggregation calculator with the new name)

I run the tests on Jenkins (http://ci.openquake.org/job/zdevel_oq-hazardlib/26/) and all seems ok.