Added answer distribution page.

This PR adds a per problem part answer distributions, which displays a chart and table of student responses.  It doesn't include navigation to get to this page (future PR when course structure for performance sections are incorporated).  Most of the copy is as specified by @lamagnifica with pending updates recorded in AN-4243 to be completed in a later PR.  The help link will be updated in a later PR too and recorded in AN-4242.

![answer-dist](https://cloud.githubusercontent.com/assets/5265058/5530599/e0df1174-89ef-11e4-9e3c-1f11c32366f1.png)

@rocha @clintonb 
At some point, I prefer if we return an object or a structure, instead of multiple values. I facilitates documentation and improves readability.

One refactoring I would do (but it is not required for this PR) is to create an `AnswerDistributionEntry` object, and move the logic of this function to its constructor.