Add health_check to aws_elb resource

I'm not sure I've done this right, but this IS working if you add mitchellh/goamz#71.

There can only be one health_check per ELB, but I can't work out how you can assert that there is only one, and that you don't need an array.

Also, I will probably need to rebase this PR if you merge #103 (add subnets and internal to the ELB resource), since it affects nearly the same lines.
Yea, right now with how we handle embedded objects, this is the best we can do. Extra health checks will just be ignored, which is fine.