Trbm/hints pr

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).
General Notes about this test file:
1) The new hinting mechanism introduces to the XML format a 'schema' attribute on the <problem> element. Thus, with every XML string produced by the new code, this new attribute must be included with the proper value ('edXML/1.0' at the time of the original PR). The tests in this file were all modified to expect both the new attribute and this initial schema value.
2) These tests, as written, are somewhat fragile in that a simple string comparison is performed between the actual resulting XML string against the expected XML string. XML parsers are designed to ignore nearly all whitespace--two XML strings can differ wildly in their use of whitespace and still be exactly equivalent in terms of the XML content of each. Considerable time was spent tweaking the new XML generation code to produce *exactly* the same XML strings so that the regression tests here would not need editing. Unfortunately, in some cases the cost of matching the expected string verbatim was deemed too great and this test file was changed instead. The justification in each case where the test was modified is the same: whitespace may be removed or added to an XML string without changing the XML content in any substantive way.