Added streams to stop files from being uploaded if any files in that fol...

...der exists.

If a file does exist in s3, pipe a null variable to gulp-s3 in which it will fail and generate an error. Once it does, nothing will get uploaded.

This is not final, code still requires cleanup.
With this, there's at least 3 variable naming patterns in this change: `duplexStream`, `ReadableStream`, `publisher_test`.  There's some semantic naming to the different styles - e.g. I would expect `ReadableStream` to be a class - but mostly I dislike the inconsistency.  (`SandboxedModule` might break some of the patterns.)