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
the \0 here and in several lines below is not needed.

Also I don't like to use strcpy and strcat; I've spent time removing them from raptor.  You can use raptor_stringbuffer to build up strings in a safe manner.