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).
Actually the new code does not do what the old code did. Briefly, the old code returned out as soon as it was determined that no hintgroup exists which is tantamount to deciding there is no 'hint_fn' function to execute. The extended hinting mechanism doesn't come into play until *after* the hintgroup determination has been made because we wanted to allow a simple mechanism to supersede the extended hints with a custom hint_fn function. So, where the old code returned out I need the new code to go on and check for any existing extended hints.