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/JsonVariantDefault.hpp

@@ -0,0 +1,26 @@
+// 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
+
+namespace ArduinoJson {
+namespace Internals {
+
+template <typename T>
+struct JsonVariantDefault {
+  static T get() {
+    return T();
+  }
+};
+
+template <typename T>
+struct JsonVariantDefault<const T> : JsonVariantDefault<T> {};
+
+template <typename T>
+struct JsonVariantDefault<T&> : JsonVariantDefault<T> {};
+}
+}