Mkr serializer

Re-implementation of mKR serializer using raptor_serialize_turtle.c and raptor_turtle_writer.c
context->emit_mkr = 1 for mkr, = 0 for turtle in raptor_serialize_turtle.c
Ditto for new emit_mkr argument for _prefix and _base procedures in raptor_turtle_writer.c
This should not be checked into GIT.