Added multi-column unique constraint, some fixes

* Added composite unique constraints via unique_together attribute
* Fixed issue where rom would leave extra data around for deleted entities
* Added convenience function util.clean_old_index() for cleaning out extra data
  for deleted entities
* Added convenience function util.show_progress() for watching progress of
  util.clean_old_index() and util.refresh_indices()
* Fixed tests should no longer leave testing data in DB 15 (running tests will
  clean out this testing data)
* Fixed bad string docs