lml/ost#345: Add questions with links to exercises section.

- Add `family` scope and method to the assignment model.
- Add `_questions` partial to show the exercise questions.
- Add `show_url` param and ability to the `_question` partial.
- Add `authority?` view helper function.
- Use `authority?` view helper function.
- Fix copyrights.

TODO:

- The styles for questions are a little weird.  Leaving things
  as is to wait for the UX review.
I wanted to ask you ( @Dantemss / @jpslav ) about this.  Would it be okay to use something like:
http://rubygems.org/gems/memoist at the helper level for this?

I was not very sure about caching, ownership and lifecycle of the helpers.   Do they live and die with a request?