Made index on exercises [course_id, name] unique.
Attempt to fix #108 by setting Capybara's default wait time 2s -> 10s.

This should make Capybara wait longer for the comet message
to appear before failing.
Merge remote-tracking branch 'mluukkai/master'
Somewhat nicer presentation of exercise completion status.
Refactored and optimized exercise completion status.

- Moved all querying from the controller into the helper.
- Got rid of N exercise object queries.
- Converted ExerciseCompletionStatusGenerator's test from
  stubs to factories.