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 
Other presenters return at most 4 values it seems, but additional values are nested inside of a `summary` dict.  The answer distribution doesn't have a summary though, which has resulted in so many return values.

The idea of an `AnswerDistributionEntry` class is interesting.  Wouldn't it subsume the majority of the presenter logic?  I can see it being more OO though.