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 

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It's really up to you. If you include the parens in the translation string, it is possible that translators will remove them. It is also possible - although I really don't know - that other languages prefer to use symbols that aren't parentheses.

I guess I would lean towards leaving in the parens. You could ask in the translators note for translators to leave in the parens or an equivalent symbol - but it's really up to you guys and how you want the UX to appear.