Added custom implementation of strtol() (issue #465)

`char` is now treated as an integral type (issue #337, #370)

include/ArduinoJson/Polyfills/isInteger.hpp

@@ -0,0 +1,22 @@
+// Copyright Benoit Blanchon 2014-2017
+// MIT License
+//
+// Arduino JSON library
+// https://github.com/bblanchon/ArduinoJson
+// If you like this project, please add a star!
+
+#pragma once
+
+#include "./ctype.hpp"
+
+namespace ArduinoJson {
+namespace Polyfills {
+
+inline bool isInteger(const char* s) {
+  if (!s) return false;
+  if (issign(*s)) s++;
+  while (isdigit(*s)) s++;
+  return *s == '\0';
+}
+}
+}