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
It's hard to read that this is the case since it's freed on lines 788, 789 but used in later code starting at line 839-851.  It's not clear this is safe.

This will probably be found by later testing by me when I do leak checks and run it through valgrind, cachegrind and static analysis tools (clang, coverity).  I expect more bugs.

(Also: is the commented out code on lines 796-798 necessary?)