Patch extensions

Some changes I made because you accepted my last patch. *podcast* is straightforward. *dc* is a bit more invasive, basically I put the extension part in one function *_extend_xml* and called it four times.
You definitely should take a look at it though, I copy&pasted your code, and the last part (lines 63-69) doesn't make sense because *format* and *identifier* aren't set anywhere. I think this should be part of the loop right above, but I don't know what you intended there.

I also don't know if the same extensions can be applied to feed/channel and entry/item, or atom and RSS for that matter, but it should be quite easy to fix this if needs be.