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
getLanguageTag -- OK

Optional -- I'd like to see how it works in practice.  The theory is fine but if application code gets messy then it may be better to be more pragmatic. This is a style thing, not a technical issue.