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.