Create grade book task.

Created a task that will generate a grade book, a CSV where the rows are
students and columns are the students scores for various assignments.

bundle exec rake 'db:query:gradebook[1]' > gradebook_course_1.csv

The above will create a grade book for the course with id 1.

Closes #686.
Merge pull request #688 from UM-USElab/gradebook

Create grade book task.