Support of the oq-lite command-line tool

See https://bugs.launchpad.net/oq-engine/+bug/1386665.
The tests are running here: https://ci.openquake.org/job/zdevel_oq-commonlib/161

There is also a companion on the engine: https://github.com/gem/oq-engine/pull/1570
Yes, since AccumDict is a mutable object, we don't want use it as a default argument. You can find tons of discussions on the net looking for "mutable default arguments python gotchas" or something like that. For instance the first one I see is here: http://stackoverflow.com/questions/1132941/least-astonishment-in-python-the-mutable-default-argument