The hire of labor or services is the essence of every bailment in which goods are delivered to another and compensation is paid for the care, attention, or labor bestowed upon them. It includes the contracts of forwarding and commission merchants, factors, keepers of wharves, mechanics, and all agents in such transactions. In all such cases, the bailee is not only bound to exercise skill in the labor and work bestowed but it is a part of his contract that he shall exercise ordinary care and diligence in keeping and protecting the articles entrusted to him.