Corrected all rdf::triple and rdf::quad forward declarations.
Switched over to using Clang as the default development compiler.
Fixed the pkg-config hyperlink in the README.
Added the XQilla library as a build and runtime dependency.

See: http://xqilla.sourceforge.net/
Implemented a simple wrapper header around XQilla's machinery.
Ensured that XSLT exceptions are uniformly thrown with xslt_error.
Ensured that xslt_processor#parse_from_url() can be called repeatedly.
Implemented the xslt_template#execute(FILE*, FILE*) method.