LDAP auth

Adds simple LDAP authentification. Uses additional config:
auth_backend: use "LDAP" to active LDAP authentification
ldaphost: LDAP server hostname
ldapport: LDAP server port
ldaprdntpl: user RDN template (%login% replaced by actual login), example: uid=%login%,ou=people,dc=example,dc=org

Just a general quesion (I'm not familiar with the LDAP specs): why the `preg_match('/[^a-zA-Z]/',$login)`?