Use html5lib for parsing if available.

For me this fixed problems in coursera_dl.parse_syllabus():
* The TestSyllabusParsing unit test failed.
  ./runtests.sh
* I could not download resources for the fe-001 class.
  ./coursera-dl -n --skip-download fe-001

After playing with BeautifulSoup for a while, I noticed that the parse tree
for the syllabus was incorrect. I have both the lxml and html5lib packages
installed, and BeautifulSoup was picking lxml automatically. When I changed
this to explicitly use html5lib, the parse tree was correct and the problems
disappeared.