clear sessions

This should cover the answer in http://lists.openmicroscopy.org.uk/pipermail/ome-users/2014-June/004528.html

Documented in https://github.com/openmicroscopy/ome-documentation/pull/834
The issue of stale session elements is not limited to the file backend. You might change the documentation here to reflect that and also provide a link to the following as a reference:
 * https://docs.djangoproject.com/en/1.6/topics/http/sessions/#clearing-the-session-store