Add heartbeat option defaults & README documentation.

`heartbeat` is a very important setting that shouldn't be ignored. Without it, clients that die will hold onto unacked messages indefinitely, until the connection is manually reaped.

This PR properly sets the `heartbeat` option to the [RabbitMQ default of 580](http://www.rabbitmq.com/configure.html#config-items) and documents the option in the README.