Use html5lib with bs4 if available.

* Explicitly request html5lib in the BeautifulSoup constructor.
* Add instructions for installation of html5lib to the README

Beautiful Soup 4 (bs4) can use any of three parsers: html.parser (builtin),
lxml, or html5lib. It seems that for at least some Coursera classes (fe-001
for example), only html5lib produces an acceptable parse tree. However, if
both lxml and html5lib are installed, bs4 will pick lxml by default. This
patch ensures that html5lib is used if it is available. These changes should
not affect users of Beautiful Soup 3.