Enable Python SKOS objects to be converted back to an RDF Graph

The `RDFBuilder` class has been added to provide this functionality.
Ensure the `RDFLoader` creates properties as strings

Properties for `Concept`s and `Collection`s were being created as RDF
Literals which was causing problems with the equality operators.
Add support for obtaining values from Dublin Core `/terms/' namespace

`ConceptScheme` and `Collection` preferentially get their title,
description and date from the Dublin Core `http://purl.org/dc/terms/`
namespace, falling back to `http://purl.org/dc/elements/1.1/` if
nothing is found.
Ensure RDF sources containing unicode symbols are properly handled