add support for downloading from S3 to a local directory

The patch turns the plugin into a full sync client, which can operate local -> S3 and S3 -> local. 

The code is changed so that the default is uploading, to make sure that existing builds will not break. By using action = 'DOWN', you can overwrite this and download from S3 to a local directory.

I also added an example in the README.md.