Implement AWS as a thin wrapper on aws-sdk

This is a major PR. It removes the home-grown aws implementation, and thinly wraps `aws-sdk`. 

There are some minor changes as well, leading up to the `v1.0` of pkgcloud.

1. `client.upload` and `client.download` are purely streaming interfaces, no more `options.local` or `options.stream`.
2. `client.destroyImage` properly deregisters the AMI before deleting the snapshot

There's still a lot of API cleanup to do, but this is far enough along to merge to master.