Fix the generated epub file to be both more efficient and compliant

* Fixed the file so that it has the correct magic bytes at offsets 30 and 38.
* Made the generated epub ~ 400K smaller (over 50% reduction).

For details/reference, see:
* http://www.ibm.com/developerworks/xml/tutorials/x-epubtut/section4.html
* http://old.idpf.org/ocf/ocf1.0/download/ocf10.htm#_Ref134878301