Implement parse_time_range against tests
Use Pingdom.stats, not stats_for_24_hours

Now we're parsing out a start and end time, we use Pingdom.stats() which
takes a start and end, rather than stats_for_24_hours() which only takes
an end date.
Remove obsolete stats_for_24_hours method

It was a wapper around `stats()` which we now don't use.