Return url from method so that it could be used on the client

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. (I think that moving forward the callbackFunction should be made optional as well)