Ref #18 - Corrections for slowest_actions_by usage

- Updated the README to reflect the new, proper usage of
  Reporter#slowest_actions_by
- Changed actual usage in the mailer to reflect the new, proper usage
- Removed a comment from the ReporterTest class since it's not actually
  necessary. There is only one optional parameter to the
  Reporter#sum_counters_by method, so there's no need at this time to implement
  an options hash.