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

- [x] Review from @willwhite @rclark 
- [x] Document required `bucket` key in `.cfnrc`

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### Getting shared secrets on first run

To address getting shared secrets on your very first launch/config of your new service, I've put a `starter.cfn.json` in our `cfn-configs` bucket. e.g. you can use it manually like this:

    cfn-config -t unpacker.template -r us-east-1 -n uptest -c s3://cfn-configs/starter.cfn.json

And have the shared values for `SumoLogicEmail`, `SumoLogicPassword`, `LibratoEmail`, `LibratoAPIKey`. I don't think we need any coded "inheritance" or global values system besides something like this for now.