Add heartbeat generator

Story: https://www.pivotaltracker.com/story/show/82210230

For monitoring purposes, a heartbeat message is sent to the published_documents exchange.
These messages have a timestamp and the hostname of the machine, along with a header of "application/x-heartbeat".

This adds a cron job running every minute to emit the heartbeat messages.
How does this combine with the `rake heartbeat_messages:send` in the `job_type` definition?