Add result string 19 Constraint Violation

As documented in http://www.openldap.org/doc/admin24/appendix-ldap-result-codes.html

Author: esamattis

lib/net/ldap.rb

@@ -322,6 +322,7 @@ class LdapError < StandardError; end
     14 => "saslBindInProgress",
     16 => "No Such Attribute",
     17 => "Undefined Attribute Type",
+    19 => "Constraint Violation",
     20 => "Attribute or Value Exists",
     32 => "No Such Object",
     34 => "Invalid DN Syntax",