Ensured that rdf::writer callbacks are passed smart pointers.
Ensured that rdf_writer_for() calls abort() on never-reached branches.
Imported a TriX-format example file for use in unit tests.

Source: http://swdev.nokia.com/trix/examples.xml
Implemented some basic unit tests for the TriX reader.
Implemented an initial version of the XSLT writer.
Switched the RDF/XML serializer to 'rdfxml-abbrev' mode.
Imported a public-domain JSON serializer from the librfc project.

See: https://github.com/dryproject/librfc/blob/master/src/rfc/rfc4627/
Cleaned up the coding conventions in <rdf++/writer.h>.
Ensured that rdf::writer has all rule-of-five operators defined.
Ensured that rdf::reader has all rule-of-five operators defined.