'Invite selected users to join the site' added as an admin action.

Fixes #20.

For more information on admin actions, see:

https://docs.djangoproject.com/en/dev/ref/contrib/admin/actions/

The state of whether or not an imported user has been emailed can also
be seen in the User model admin page, though it needs to be
un-collapsed.
added __unicode__() for Membership objects.
added __unicode__() for ImportedUserInfo objects.