Ref #22 - Fix counters in email template

- The previous code assumed that the email view would receive an array of
  HowSlow::Metrics::Counter instances, when in fact it was getting an array of
  Fixnum.
- This was just a blantant bug. The proper information was never being passed to
  the view, so I fixed that by having `@counter_metrics` return an array of
  arrays, where each array contains two elements: first the name of the counter
  being counted, and second its actual count.
- Updated the view to reflect this.