Extended Hinting

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).
I would recommend putting this schema string in a variable at the top of the test file and referencing the variable throughout. That way the test can easily be updated if the version is changed in the future.

Better yet if the schema variable could be defined in the code file where it is used, and then referenced in this test file. This would mean that the test itself would never need to be touched.