Njdupoux/proxy bug fix

Fix for proxy bugs stemming from improper html sanitization

In order to prevent improper html being sent back from the grader and proxy due to lack of escaping of characters such as "&", I've cleaned up the html formatting code to cleanly sanitize the html feedback response received from the grader. This includes proper html escaping. 

I've also written a few units tests to test the sanitization, and added some documentation 
Ah, but then we're parsing this `lxml`.  So perhaps your modifications are required to make it work?  I can see `<br>` not being valid xml.