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 
Some of the logic here, and in lines above, assumes that `problem_part_id` is valid and part of the question. If it is not, the code here would be `problem = [][0]` which will raise a 500 error. Is there a point somewhere else that we confirm the value of `problem_part_id`, and if it is not value return a 400 instead?

Minor: What does 'i' stand for? Why not `q`?