new visibility state; started writing unit tests

moved Evaluation.evaluation_type to ReadingSystem.visibility
only superusers can make a reading system publicly visible
removed EvaluationForm as it was no longer useful
added visibility management to /manage/ page
fixed scoring bug in index page (was reporting N/A instead of 0% for
brand-new reading systems)
added permissions.py module
added corresponding custom assignment tags for permissions

first pass at unit testing (not working; suspect an issue with
django_evolutions DB manager add-on)
added test content EPUBs to be used in unit testing

simplified import_testsuite_data main script args handling

added more documentation