Finish 1.0.8
Speed up access to RDF::x URI constants.

Previously accessing these unnecessarily created a new (interned) URI,
which is relatively slow.
Make literal initialisation a little faster.

This is mainly from comparing datatypes by string rather than by URIs,
which are slow to initialise.
Add Tom Nixon to credits.
Merge branch 'tomjnixon-speedup_literal'
This closes issue #129
Add schema.org vocabulary as RDF::SCHEMA.
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
Update specs with expect syntax.
Finish UpdateSpecs
Release 1.0.9
Finish 1.0.9