ec2:AllocateAddress

I've added some functions for allocating and associating elastic IPs (and disassociating and releasing).

I've been a bit lazy and it only applies to VPC instances (not EC2-Classic), but I don't think it's much needed for EC2-Classic.

I needed this for an addition I made to packer so it could get into my VPC instances to provision them. If you're happy I can submit a pull-request for that too. Having said that, I'm under the impression that packer will SSH into the AWS instance using either the DNSName (on ec2 classic) or the private IP on VPC (assuming that a VPN is set up). So I thought this would save setting up a VPN. Is that right? See https://github.com/sorohan/packer/compare/allocate_elasticip_patch.