Created JsonSerializable (previously Jsonable) and JsonDeserializable.
All relevant classes now subclass JsonDeserializable to eliminate unneeded json -> string -> json conversions.
Changed apihelper#convert_markup
Constructed the Jsonable abstract class. All subclasses must override Jsonable#to_json.
Made ReplyKeyboardHide, ReplyKeyboardMarkup and ForceReply a subclass of Jsonable to make things less complicated in convert_markup.