Ensured that the rdf::triple and rdf::quad constructors/destructors
are declared in-line in the header files.
Fixed compilation errors in <rdf++/triple.h> and <rdf++/quad.h>.
Cleaned up relative header inclusion directives.
Ensured that <rdf++/{triple,quad}.h> include <rdf++/term.h>.
Added stubs for upcoming RDF/JSON format support.
Simplified the unit test directory structure.
Cleaned up coding conventions in <rdf++/term.h>.