Issue 1331: Account creation options

Resolves issue: http://code.google.com/p/otwarchive/issues/detail?id=1331

Created the 'creation_requires_invite' setting on the admin_settings table. If account creation is enabled AND 'creation_requires_invite' is enabled then the user is given a message saying that they will need an invite, and are deposited on the Invite queue page. 
+1 for removing this ;)