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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One benefit of leaving the number in the text is that it would be easier for translators, see [singular vs. plural](https://github.com/edx/edx-platform/blob/master/docs/en_us/developers/source/i18n.rst#singular-vs-plural).  The example provided is:

```
msg = ungettext("There is {file_count} file", "There are {file_count} files", count)
```

We could do perform this logic in the view and pass the tooltip to the template.  It seems perhaps more complex than necessary though.