Fix open potx

[y.rolland]

Hi Jason,

I've had a problem for opening potx with pptx4j. In fact, there was an error (/ppt/presentation.xml unreconized part). I think it's due to the addition of the presentation part when the file is a potx (in JaxbPmlPart, the function newPartForContentType add a part only if it's a ContentTypes.PRESENTATIONML_MAIN and not a ContentTypes.PRESENTATIONML_TEMPLATE).
I submit you a patch with this change, but I don't know if it could cause failure on some cases.

Thank for the great work! I'am using docx for two months now and I really appreciate it!!