Changes for Writer validation and canonicalization.
* When writing, if the :validate option is true, raise an error when attempting to write any statement which is invalid.
* When writing, if the :canonicalize option is true, attempt to canonicalize each statement when writing. Raise WriterError if any canonicalization attempt fails.
* Canonicalizing a statement only attempts to canonicalize terms which are not frozen.
* Move default #canonicalize and #canonicalize! to RDF::Value.
* More migration of RSPec patterns to 3.0 expectaion behavior.
* Improve statement validation to require Resource for subject and context, URI for predicate and Term for object.
This addresses issue #126.
Default validation on for NTriples/NQuads writers.
Fix odd double canonicalization error for e.g. 1.1E-01 => 1.1E-1
Release 1.0.8.
Finish 1.0.8
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.
Merged 'develop'.