Update Countable#enum_for to use Countable::Enumerator rather than ::Enumerator extended with RDF::Countable.
This parially addresses performance issues noted in issue #123
Implement Enumerable#enum_statement using Queryable::Enumerator, which includes Queryable and Enumerable; this avoids having to extend #enum_for
This relates to issue #123
Change result of RDF::Queryable#query to return a Queryable::Enumerator insttead of just an Enumerator, this avoids neading to extend the enumerator.
This relates to issue #123
Finish CustomEnumerators