When possible, don't use a namespace prefix

Some media players (like VLC) can't parse the playlist correctly if the XSPF tags are prefixed with the autogenerated namespace `ns0:`, even if it's semantically correct.

The [function in question](http://docs.python.org/2/library/xml.etree.elementtree#xml.etree.ElementTree.register_namespace) was introduced in Python 2.7.