Call functions with multiple argument, not an array

To keep it consistent with the [PhantomJS API](https://github.com/ariya/phantomjs/wiki/API-Reference-WebPage#wiki-webpage-onError), I propose calling functions with separate arguments, not a single list of them.

For example, using the current code, the following code is necessary:
```javascript
page.onError = function(error) {
  var errorMessage = error[0];
  var stackTrace = error[1];
  console.log(errorMessage);
}
```

My proposed change would allow
```javascript
page.onError = function(errorMessage, stackTrace) {
  console.error(errorMessage);
}
```

Anyway, thank you for your code!  It is the best node-phantomJS interface that I've found so far!