Session07 - html renderer

I implemented this and you can see my tests including the one above in test_html_render.py.

My implementation throws an AttributeError if someone tries to put something that is not an instance of OneLineTag or an instance of a child of OneLineTag because other classes don't have my new method to render onto one line.  But, it doesn't make sense to put something that can go on multiple lines inside a OneLineTag though, so it's probably fine that way. I will make a pull request as soon as I can reach Github again.