add ec2.VpcAttribute and ec2.ModifyVpcAttribute

Does what it says on the box. I tried to keep the same idioms as the `ModifyInstance` function with the `Set...` fields. I think, long term, that they're a bad idea, but that's not a matter for right now. I don't know quite how to add tests, so I'd love it if someone gave me a bit of a rundown on how to go about doing that.