MathJax Integration

Small tweaks before merging in
https://github.com/edx/edx-ora2/pull/617

@wedaly @andy-armstrong I made some small changes to fix tests, etc. Just tested this on devstack and it looks good to go. Let me know if it looks good to you, and I'll merge it in

@sjang92 Once I get an internal review I'll merge this to master. It should get into the next release. 
Oops I think this line should be taken out. MathJax rendering is being done in oa_server's render() method now.