More work on Python 3 support

Building on @philipkimmey's work in [this pull request](https://github.com/roverdotcom/django-inlinecss/pull/12), I've switched django-inlinecss to use BeautifulSoup 4, and fixed many minor problems.

The tests now run on at least Python 2.7 and 3.3 without _errors_, although there are many _failures_.  I think this stems from the changes to BeautifulSoup.

Some of the failures seem trivial, like re-ordering of html attributes.  Some are bigger.

It's obviously not ready to merge into master yet, but I wanted to share my start on the work.