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
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.
Merge branch 'master' into bye-bye-localization

Conflicts:
	html5lib/treewalkers/_base.py