New Hinting Mechanism

This pull request represents an implementation for a new hint (aka targeted feedback) mechanism designed to provide a coherent approach to hinting in all of the five most frequently used problem types: checkboxes, multiple choice, drop down, numeric input, and text input. There are four parts to this new mechanism: 

1) the documentation describing how the new hint material will be encoded in the XML representation of problems (see https://docs.google.com/document/d/1h1cWx24VWkYCdwQEV8rY2Jm1xcZZp6_EQQbzDqbrnks/edit#heading=h.obezscv16esk), 

2) a description of the new syntax added to the simple editor's markdown parser to allow a user of that tool to capture the hinting information in a way which will produce correct XML (see the link above),

3) the code changes which were made to the markdown parser (see problem/edit.coffee),

4) the code changes which were made to the response processing to extract the new hint material from the XML representation and add it to the html presented to the student (see responsetypes.py).
@sarina Hey Sarina. my take is that the OLI people have a vision (and coding time!) to really advance feedback. I talked with them about what I didn't like about 1499 back when this was getting designed, and I like what they have better than the 1499 form.

This PR leaves the 1499 form in place. I think a good followon step would be to translate the 1499 form to this form at xml-load time, and then we can take the 1499 implementation out. This thing is so big, I wouldn't add that onto this PR.