Support elbv2?

Hello. I found that letsencrypt-aws doesn't work when the ELB type is application, since the new ELBs can only be accessed through [elbv2](http://docs.aws.amazon.com/cli/latest/reference/elbv2/index.html). It is a good idea to support both type of ELBs.

I can submit patches when I figured out how...