Refactor assigning graders

This patch refactors the signatures of `Grouping.update_assigned_groups_counts` and `Grouping.update_criteria_coverage_counts` so that they have a consistent style. This is in preparation for a following patch that improves the performance of assigning graders to criteria, which makes use of the new signature of the two methods.

This patch also updates the current Test::Unit tests and migrate them to RSpec.

Tested:
- rspec
- rake test:units
- rake test:functionals
- Ruby 1.9.3, 2.1.2, Postgres