Consistent error handling

[vogdb]

Hi! What do you think about changing https://github.com/maxogden/github-oauth/blob/master/index.js#L45 from

if (!code) return emitter.emit('error', {error: 'missing oauth code'}, resp)

to something like this:

if (!code) {
  var err = {error: 'missing oauth code'}
  if (cb) {
    cb(err)
  }
  return emitter.emit('error', err, resp)
}

because now if I rely on a callback that is the case when my callback won't be triggered.