Replace http://unknown.org for unkown terms

[mdoering]

For terms that cannot be mapped to any existing term enumeration the term factory creates an UnknownTerm instance. If no absolute URI with an authority is given, simple names are placed into the http://unknown.org namespace.

This is confusing for users. We also do not own the domain. It would be best if these terms could live just as plain simple names without a namespace and the qualified name being equal to the simple one.