Correctly implement XML namespace parsing

[farski]

I think part of the best-practices documentation we produce should include language that helps ensure people who are targeting syndicated.media conformance parse namespaced XML elements correctly. Ie don't do look for itunes:image, use XML parsers' support for something like find('image', 'http://www.itunes.com/dtds/podcast-1.0.dtd').

This has already come up in #11 and I think we all have experience with apps that do it incorrectly and make things tough.