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!
Some comments would not be frowned upon.  Please explain WHY this is being done.