Release 0.2.0

* Perform entailments accross Statement, Enumerable, and Mutable in addition to Vocabulary::Term.
  * e.g., `graph.entail` now returns a new graph including inferred statements using all entailments in a single pass.
* Add rdfs:domain, rdfs:range, owl:equivalentClass, and owl:equivalentProperty entailments.
* Allow plain literals to be coerced to datatyped values, not just simple literals.
* Added OGP datatype validataion.
Version 0.2.1.
Release 0.2.1

* Allow plain literals to be coerced to datatyped values, not just simple literals.
* Added OGP datatype validataion.