Possible changes 

Possibilities for discussion.

1. Quads are not extended Triples (useful if the default graph is a union of named graphs but this isn't always the case) but share a common supertype.
1.  Add a CONTRIBUTING file
1.  3 parts of a Literal
1.  getObject is a Resource, not a Literal
Changed `asTriple` to `toTriple` (c.f. `toString`) - hope that's clearer.