Automatically register/deregister with Route53 for round-robin DNS balancing

[Castaglia]

For some sites, having mod_aws automatically register that instance's IP address with Route 53, as e.g. an additional A record for a DNS name as part of a DNS round-robin balancing scheme, would be very useful. On server shutdown, mod_aws would de-register itself from Route53 as well.

This would also mean implementing an HTTP "health check" listener, for Route53 to occasionally "ping" to make sure that the instance is still available/ready for connections.