Remove parsed source

This is another simplification-oriented pull request. It removes the table parsed_source from the database. Now the sources as sent to the workers directly via rabbitmq/celery. Several performance analysis were done, both on a laptop, on gemsun04 and on the cluster. The result is that there are no performance regressions and the memory is under control. I tested also the USA source model, with 256,000+ sources, one of the biggest we have and it runs even on a laptop.See http://performance.openquake.org/jobs-2013-11-26.html for the run time on the cluster (comparing the current master with this branch).

Test running on http://ci.openquake.org/job/zdevel_oq-engine/193. Notice that all demos are slightly faster.