Fix #78: drop RecursiveTreeWalker.

Nothing has used this for years, and it shouldn't be used as it can
easily run out of stack (and a non-recursive algorithm is scarcely
more complex).
Move import/constants to top of file to match style.
Bump version number to 1.0b3.
Add changelog for 1.0b3 (uh, this ought to be have done before).
Bump version to 1.0b4
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.
Push this as 0.99, after PEP 440 pushed pip to ignore pre-releases.

There are no significant changes in this compared with 1.0b3 (the
only changes are documentation/testsuite related).
Increment version number after 0.99 (provisionally 0.999).
Merge remote-tracking branch 'origin/master' into escape-characters-serializer