Bring main callback to node.js error conventions

Bring main callback to node.js error conventions instead of having both error and response data in one variable, because actual response is not an error.
http://docs.nodejitsu.com/articles/errors/what-are-the-error-conventions

So the actual call would look like 
```javascript
sms.sms_send({
  to: '79112223344',
  text: 'Текст SMS'
}, unction(error, response) {
    console.log(error, response))
})
```