Return the url value from the s3upload method 

This way you can supply an additional callback method to be executed in the front-end which would receive the url.

i.e
```
// Client-side
Meteor.call("S3upload",fileData,context,callbackFunction, function(err, url){
  // do something with the url server side
});
```
Note the callbackFunction is still the name of the method that will be executed server-side, not the asyncCallback that's called automatically by default.