added ldap authentification to Rdf auth adapter

[FTeichmann]

To enable ldap authentification in Ontowiki, import these users into the System User Graph.

e.g. using https://github.com/FTeichmann/ldif2OntowikiUsers but the process is basically:

• add a predicate 'sysont:authenticateViaLdap' and set it to "true"^^xsd:boolean for the specified ldap user
• Virtuoso 6.1 AND Virtuoso 7 convert boolean true to "1"^^xsd:integer when saving the model and the Erfurt auth adapter reflects that behavior
• add a predicate 'http://www.w3.org/2007/ont/ldif/dn' to the literal of the ldap user dn as plain string
• configure the ldap connection in the respective chapter of the Erfurt configuration file