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 
I wouldn't use the _ prefix for the methods above. They are normally considered `private` and not use outside the class. I think here you are trying to signal that they are `protected`, but that concept is not really used in python.

Another point is that this methods don't really have to be inside a class, they can be module level functions. They are not using any class specific values, and are in fact very general purpose. Putting them in a `util` module could be useful but not required.