Adds HeartbeatGenerator and GovukExchange

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 work is performed by the `HeartbeatGenerator` class.

This PR also adds `GovukExchange`, which tries to encapsulate how we
write to exchanges at the moment. It also allows for a shorter setup for
publishing messages, useful for Rake tasks or cron jobs.
Add hearbeat_messages:send Rake task
Add routing to heartbeat messages
Corrects rake job_type setup
Stop exchange name from being pushed to Bunny
Merge pull request #73 from alphagov/add-heartbeat-generator

Add heartbeat generator

https://www.pivotaltracker.com/story/show/82210230