Simple ELB support

Adds functionality of adding and removing instances from AWS ELB.
ELB must exist in the first place.

This partially fixes mitchellh/vagrant-aws#53
@sevos There is an issue with this behaviour in VPCs (described here https://github.com/mitchellh/vagrant-aws/issues/250)

> Load balancing between availability zones is made on DNS level.

I've not found this to be true, I have several ELBs (in VPCs) in production that are in multiple AZs but only have one instance registered and have never encountered any packet loss. Can you clarify where you're seeing this issue?