Create checks for pingdom credits and aggregates.

For the bounty advertised in the
[mailing
list](https://groups.google.com/forum/?fromgroups#!topic/sensu-users/n8qT5zkK3t4),
based on
[these
requirements](https://gist.github.com/nstielau/766d50faad7629997751).
I got shot by rubocop!
Merge pull request #221 from webmat/pingdom_bounty

Create checks for pingdom credits and aggregates.