Elastic IP for VPC instances per issue 18

I needed this functionality so I wrote up part of what you discussed in Issue #18

> A configuration aws.elastic_ip = true

Done
> When set, part of the creation process creates an elastic IP (a new one) and assigns it to the instance.

Done

> Some changes needed to make it work:

> Provider ssh_info needs to take public IPs into account as a potential SSH candidate.

Done
> Elastic IP needs to be tracked in the machine data dir and released when the VM is destroyed.

~~Not done. Not sure what the best way to accomplish this is.~~
Added in bwhaley/vagrant-aws@69a1f4fbef2c6f4a479eafcb14f08767a2ab9987