S3 file loading

Implemented this at at the generic json config file loading level so both templates and config files can be loaded using `s3://` URLs.

This now works:

```
cfn-create -t cloudformation/osrm.template -r us-east-1 -n will-test -c s3://cfn-configs/osrm-api/staging.cfn.json
```

/cc @rclark @yhahn