Update html5lib to 0.999 (and Python 3 support)

Update html5lib to 0.999 (as released on 2013-12-23); applied to the Python 3 support added by others.

This Pull Request (PR) is heavily based on previous work by @marcdm and @msabramo (see #101 *"Py3k and html5lib 1.0b1 support"*); now updated to the html5lib 0.999 (as released on 2013-12-23). See https://github.com/html5lib/html5lib-python/blob/master/CHANGES.rst (and especially the 0.99 section) on the rationale of html5lib to change their version numbering like this (going back from 1.0b3 to 0.99).

Tested on py26, py27, py32, py33, and pypy using Travis-CI, see https://travis-ci.org/mdxs/bleach for the results.

This PR could: supersede and close #101 *"Py3k and html5lib 1.0b1 support"* (and this PR also applies the @dstufft feedback given there); close #100 *"Python 3 and html5lib 1.0 compatibility"*; close #110 *"Support html5lib>=0.99"*; and close #94 *"simpletree TreeWalker removed in html5lib 1.0b1"*

This PR suggests a version bump to 1.3.1 (note that https://pypi.python.org/pypi/bleach-py3k/1.3.0 is out there); and though I'm not familiair with the release procedures, I've tried to be consistent in all the "version holding" files.

Relatively new to git and GitHub, I've first submitted a PR to the fork by @marcdm (see https://github.com/marcdm/bleach/pull/3) but hope it is okay that I now also propose it to `jsocol:master` directly (it seems to include all the previous work...).