Added initial import for turtle.
Fixed a data modelling bug were we allowed properties to point to a collection of documents. Properties now point to a single literal or a single document. If you want multiple values, you assert multiple properties with the same name for each value.
Added a UML diagram for datatree.
Minor cleanup for Document.
Roundtripping with turtle-io works now.

Extended literals to cover what we actually need. Started putting in a visitor framework to get rid of casts everywhere.
Replaced cascading instanceof checks with visitor calls. This forced me to also implement exhaustive handling of literals in various places that were a bit patchy before.

RDF serialization now works correctly regardless of if the RDF namespace is explicitly registered or not.
Merge branch 'feature/turtle' into develop