Add support for Prefix key

Once again, I'm not sure what I'm doing :D.

1. How can I trim in JS using a specific character (like trim($string, '/') in PHP ?). Should I use a regex for that?
2. After looking at your code, I'm not sure about using the URL as the key. I know S3 does not "know" folder (in fact all files are stored in a giant flat way), but the console (and most tools) uses the "/" to emulate a hierarchy. So if you store "http://www.mywebsite.com/" it will likely appears like this:

```php
my-bucket
    http:
         /
             www.mywebsite.com
```
(not tested though, but htat seems logicla with the way S3 works)