docx.py -> opendocx

I modified the opendocx function in docx.py to allow it to open other xml files than just document.xml. Its now capable of opening all three header and footer xml files as well. I used this functionality to create a script that would extract text from a full document (header, body, footer) not just the body (as the example code does). I thought it was useful and I thought others might like it too. The code is backwards compatible, so if you don't specify fileType, it should work just fine.