Add comment to order

Added function to add a comment to an order.

example use: 
require 'magento_api_wrapper'
api = MagentoApiWrapper::Sales.new(magento_url: 'https://your.domain.com', magento_username: 'api_username', magento_api_key: 'api_key')
api.add_comment_to_order(order_increment_id: '200001234', status: 'complete', comment: 'some message')