RethinkDB native connector work, minor fixes.

ext/librethinkdbxx/src/json_p.h

@@ -0,0 +1,19 @@
+#pragma once
+
+#include "datum.h"
+
+namespace rapidjson {
+  class CrtAllocator;
+  template<typename> struct UTF8;
+  template <typename, typename> class GenericValue;
+  template <typename> class MemoryPoolAllocator;
+  typedef GenericValue<UTF8<char>, MemoryPoolAllocator<CrtAllocator> > Value;
+}
+
+namespace RethinkDB {
+
+Datum read_datum(const std::string&);
+Datum read_datum(const rapidjson::Value &json);
+std::string write_datum(const Datum&);
+
+}