Fix #33: Reject empty passwords by default.
Unless AUTH_LDAP_PERMIT_EMPTY_PASSWORD is set to True, LDAPBackend.authenticate() will immediately return None if the password is empty. This is technically backwards-incompatible, but it’s a more secure default for those LDAP servers that are configured such that binds without passwords always succeed.
Fix #39: Add support for pickling LDAP-authenticated users.