moved class into "nlohmann" namespace

- fixed issue #9
- also removed std::mutex member variable
- also added “std::” prefix to size_t variables

benchmark/parse.cc

@@ -2,7 +2,7 @@
 
 int main()
 {
-    json j;
+    nlohmann::json j;
     j << std::cin;
     return 0;
 }