Allow graphite checks to define handlers
Add pagerduty handler and redphone gem

Redphone is used by the handler
Alert on low disk space

I had to add the new elasticsearch mount as the server_checks only check
the space on the root partition
Alert if mongod goes down
Alert if logstash goes down
Machines going down should alert with pagerduty
Add pagerduty to mongo up check
Merge pull request #112 from alphagov/all-the-checks

All the checks