get_bound_ldapuser(): support custom username.
Replace django-auth-ldap with ldapdb-based auth backend.
Fix login tests for proper handling of LDAP exceptions.
Clean up django_auth_ldap leftovers.
Normalize usernames for LDAP.

This decreases the database pollution, and increases compatibility with
mockldap.
Add tests for LDAPAuthBackend.
Merge pull request #92 from mgorny/no-django_ldap_auth

Replace django_auth_ldap with own backend