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 
Jeff's code tried to find where truncating the feedback left mismatched tags and to pair them off.  Granted, his way of doing this was a bit clumsy.  But maybe this was important for some of the db class questions to make them still render if someone had way too much output.  Are you sure you can lose that?