Template files on S3

Adds support for template files hosted on S3 by specifying their S3 URI at command-line, for example:

```
cfn-create -r us-east-1 -n my-stack -t s3://my-bucket/my-template.json
```

A template file larger than 51200 bytes **must** be uploaded to S3 first ([per AWS documentation](http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/cfn-using-console-create-stack-template.html)). If you try and interact with a template file larger than 51200 bytes, [readTemplate](https://github.com/mapbox/cfn-config/blob/s3-template/index.js#L62-L78) will automatically upload it to a `cfn-config` bucket.

refs #16