Converted all remaining 'keep_since' uses into the term 'retention'.
Updated CHANGELOG to reflect keep_since => retention change.
Ref #18 - Reporter#slowest_actions_by now uses options hash
Ref #18 - Changed hash usage to Ruby 1.8 supported style.
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.
Ref #18 - Updated CHANGELOG to note breaking changes for this release.
Ref #18 - Bumped version for pre-release
Fixed header type in CHANGELOG
Merge pull request #28 from jefflunt/18-audit-optional-params

18 audit optional params