Use S3 as canonical store for configs

Per https://github.com/mapbox/cfn-config/issues/36, idea is to discourage persistence of configs to disk and encourage saving of configs to S3.

![out](https://f.cloud.github.com/assets/83384/2457116/d9e31ce6-af2d-11e3-8150-97cf1e8a2385.gif)

Each configs `s3://` url is fully exposed making it easy to `s3cmd` get/del whatever you need to and making it really obvious what is going on behind the scenes.

I've gone ahead and removed to-disk writing of configs -- the stdout output of the JSON though is easily copied-pasted if you still want to.

### TODO

- [ ] Review from @willwhite @rclark 
- [ ] Document required `bucket` key in `.cfnrc`