Add examples using urllib2 and urllib.request for HTTP Content-Type

I've seen many use html5lib completely ignoring any HTTP-layer given
character encoding. Would be better to lead them in the right direction.
Fix #83: Stop testing the DOM treewalker with pxdom

This hasn't actually been tested in years and almost certainly won't
work; furthermore pxdom isn't supported under Python 3 so it doesn't
provide any testing whatsoever for a strict DOM implementation there.