Fix authentication for user/group validations

This fixes issue #40.  User passwords should *always* be checked during authentication (except when a user fails to satisfy given requirements).  Previously, the PHASE_CHECK_BIND step of authentication would not check passwords in any LDAP configuration where ``require valid_user`` was not specified (eg using ``require user`` or ``require group``).
Merge pull request #41 from davidjb/master

Fix authentication for user/group validations - passwords fail to be checked