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 refactoring out a function here.  The flow would then look something like

```javascript
var object = s3(...);
object.location = ...;
var returnStream = wrapInFileExistsCheckStream(object);
return returnStream;
```

The idea is to make this method shorter and clearer by moving the specifics out to another function.