Ref. #2, #7, #9 - Started test suite rewrite

- Updated test/log/test_metrics.log to include the new counter metric
  type
- Added rake and a Rakefile so that tests can now be run via
  `bundle exec rake test`
- Added the pry gem to make it a lot easier to debug
- Added the minitest-spec-context gem to make tests much more readable
- Added the ability for the Reporter#slowest_actions_by method to handle
  a nil value for number_of_actions
- Added the ability for the Reporter#sum_counters_by method to handle a
  nil value for event_name
- Restructured the test suite folders to match the folder structure of
  the gem itself
- Rewrote the Reporter tests completely to match the new styles enabled
  by the minitest-spec-context gem