allow passing stat_name as string to Tracker

Hi guys,
there are places in Tours where we find initializing Babysitter::Tracker directly without surrounding the block by monitor being better solution. It allowed us to reuse the code and saved some of unecesarry lines while we kept the functionality you are expecting from it (reporting all important information to correct bucket).

To keep the code as readable as possible we would prefer to pass the stat_name as a string instead of an array. Here is a little fix for it.

Let me know if you have any concerns regarding using the tracker directly in code.

Robert