“Employer” also means any person contracting for the creation of a specially ordered or commissioned work of authorship when the parties expressly agree in a written instrument signed by them that the work shall be considered a work made for hire, as defined in Section 101 of Title 17 of the United States Code, and the ordering or commissioning party obtains ownership of all of the rights comprised in the copyright in the work. The ordering or commissioning party shall be the employer of the author of the work for the purposes of this part.