add replacePath() to replace fullBucketPath in server side

I added `replacePath()` method to replace `fullBucketPath`  **on upload** event.
You can use it to generate/sanitize the path to upload.


### Usage

```
// resources/s3bucket/upload.js

// fileName(e.g. 'filename.jpg') is passed from domain.
var path = Math.floor(Math.random()*10); // path = 1;

replacePath(path + '/' + fileName);

// => upload to: https://yourbucket.s3-website-us-east-1.amazonaws.com/1/filename.jpg
```