Slower worker reaper

Hi, I'm opening this in order to give you an idea of how my idea of how to solve ( https://github.com/devinus/poolboy/issues/30 ) looks.  *It's not ready*: it does not pass the tests, and needs to be configurable as well, so I'll continue to work on it and accumulate changes here, but I wouldn't mind it at all if you had a look.  Thanks.