Templatized all functions using String or std::string

* Removed `ArduinoJson::String`
* Removed `JsonVariant::defaultValue<T>()`
* Removed non-template `JsonObject::get()` and `JsonArray.get()`
* Fixed support for `StringSumHelper` (issue #184)
* Replaced `ARDUINOJSON_USE_ARDUINO_STRING` by `ARDUINOJSON_ENABLE_STD_STRING` and `ARDUINOJSON_ENABLE_ARDUINO_STRING` (issue #378)
* Added example `StringExample.ino` to show where `String` can be used

include/ArduinoJson/Internals/ValueSetter.hpp

@@ -0,0 +1,41 @@
+// Copyright Benoit Blanchon 2014-2016
+// MIT License
+//
+// Arduino JSON library
+// https://github.com/bblanchon/ArduinoJson
+// If you like this project, please add a star!
+
+#pragma once
+
+#include "../JsonBuffer.hpp"
+#include "../JsonVariant.hpp"
+#include "../TypeTraits/EnableIf.hpp"
+#include "StringFuncs.hpp"
+
+namespace ArduinoJson {
+namespace Internals {
+
+template <typename TSource, typename Enable = void>
+struct ValueSetter {
+  template <typename TDestination>
+  static bool set(JsonBuffer*, TDestination& destination,
+                  const TSource& source) {
+    destination = source;
+    return true;
+  }
+};
+
+template <typename TSource>
+struct ValueSetter<TSource, typename TypeTraits::EnableIf<
+                                StringFuncs<TSource>::should_duplicate>::type> {
+  template <typename TDestination>
+  static bool set(JsonBuffer* buffer, TDestination& destination,
+                  const TSource& source) {
+    const char* copy = buffer->strdup(source);
+    if (!copy) return false;
+    destination = copy;
+    return true;
+  }
+};
+}
+}