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 
This method does not seem to benefit from moving it to `utils` since it is very specific to how data is structured. I would leave it where it was originally, or make it more generic.