A few tweaks that should reduce the number of needed calls to heroku ps API. Heroku is rate limiting API calls to 1200 an hour which makes this gem unusable for apps with large number of jobs, but this should noticably extend it's lifetime.

I've added the specs for new behaviour and in the process cleaned up existing specs a bit.
Merge pull request #56 from radanskoric/master

A few small tweaks that should reduce the number of Heroku API calls and help apps not hit the Heroku API rate limit.