updates render callback style to follow node convention

Node callback style is cb(null, obj) on success, and cb(err) on fail – the err is not a boolean, but an error object. 

Handing an error from the render function should check for the existence of the error object, not the value, because that err should be an actual error message or error object.

```
Pdf.render('http://www.google.com', 'google2.pdf', function(err, file){
  if (err) {
    cb(err) //or res.send(err), or whatever error handler you want
  }
  //do whatever with the file
```

Otherwise, the error handler has to assume that `file