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.
Consider using `Sinon` for this instead.  In this case, you'd end up with something like:

```javascript
var dataHandler = sinon.spy();
gulp.src('./test/dist/**')
    .pipe( publisher(options) )
    .on('data', dataHandler)
    .on('end', function (err) {
        expect(dataHandler).to.have.been.called;
        done();
    });
```