show rdf:type with color or icon

 Allow to color nodes by type (eg :Person red, :Organization blue).
This is especially useful If types are hidden or inlined (#16).

Also allow to show types as icon. 

@kideneh does your PR do that?

Corner case:
- what if multiple types? 
- in particular if there is subClassOf inference, one would want to use sesame:directType (and similar special predicate for Jena)