Support for AWS ELB

Mostly for review/discussion.  I think that there are several patches here which I will need to split out.

Most trickily, AWS ELB inbound traffic has a destination of the instance IP; I think that GCE has a destination of the load balancer's public IP.  This requires some tweaks to proxying, and in order to avoid a loop the proxy needs to know the private subnet (10.0.0.0/8).  I think it could also use the AWS instance IP instead.

Other than that, no huge surprises.  I needed the Zones functionality, and so I get the current AZ from the EC2 metadata service.  I guess we should get the (default?) region from here as well.

I propose a good first step would be to add the Godeps dependency for ELB, so the patch becomes more manageable!
Thsi should be an if/else pair - it would be clearer.