implemented Connector::Builtin::Authentication::LDAP

This implements LDAP authentication. Uses Net::LDAP (not Connector::Proxy::Net::LDAP) as the authentication is quite specific (so I decided to make it independent on the proxy connector and give it its own life). It's basically verified with most typical scenarios. Would be nice, if someone could assist (or give hints) in writting regression test (if necessary and possible - I have no experience in perl test suites).