move ctors

* Add move constructor to Value::CZString
* Add unit test for Value move constructor
* Allow includer to specify in advance the value for
JSON_HAS_RVALUE_REFERENCES

include/json/value.h

@@ -212,6 +212,9 @@ private:
     CZString(ArrayIndex index);
     CZString(char const* str, unsigned length, DuplicationPolicy allocate);
     CZString(CZString const& other);
+#if JSON_HAS_RVALUE_REFERENCES
+    CZString(CZString&& other);
+#endif
     ~CZString();
     CZString& operator=(CZString other);
     bool operator<(CZString const& other) const;
@@ -294,6 +297,10 @@ Json::Value obj_value(Json::objectValue); // {}
   Value(bool value);
   /// Deep copy.
   Value(const Value& other);
+#if JSON_HAS_RVALUE_REFERENCES
+  /// Move constructor
+  Value(Value&& other);
+#endif
   ~Value();
 
   /// Deep copy, then swap(other).