Added the ability to retrieve users and groups from two separate base DN's
 - Removed the LDAP_SYNC_BASE config item and added LDAP_SYNC_USER_BASE and LDAP_SYNC_GROUP_BASE

Rewrote the sync_ldap_users method.
 - Now uses the queryset bulk_create method to create new users. This is much much faster than creating each by hand. (Has the downside that I haven't worked out how to call user.set_unusable_password() on created users. Yet.
 - Only updates existing users when the LDAP attributes will actually make a change, this is also much faster.
 - Added an option to delete or disable existing django users that are not in the LDAP store.
   - Controlled by the LDAP_SYNC_USER_REMOVAL_ACTION option, can take 1 of three values, disable, delete or nothing.
 - Added a configuration option (LDAP_SYNC_USER_EXEMPT_FROM_REMOVAL) to exempt given usernames from the sync process (wont ever be deleted).