Check plugins and other goodness

* a ``check_plugin_instances()`` command for ``cms check``
* ``general improvements to plugin_report()`` and the ``cms delete_orphaned_plugins`` command
* no more output leaks into tests from ``cms delete_orphaned_plugins`` (resolves #1590)
* ``cms.tests.check`` is improved so it can handle both databaseless and, er, databaseful tests (ojii's handiwork)

I commend this pull request to the house.