made api.observe.on method accepts callback for gmail's post response

changes in response to issue https://github.com/KartikTalwar/gmail.js/issues/54

Some informations are only available in gmail's post response data.
Like the Email id of a newly sent email, this is not available on send_message post params.

sample usage:
```javascript
function responseCallback(url, body, data, response_data) {
    var email_id = response_data[2][1][3][0];
    var emailURL = '#inbox/' + email_id;
    if (window.location.hash !== emailURL) {
         setTimeout(function() {
             window.location.hash = emailURL; // opens the thread of the email, 
         }, 200)
    }
}
gmail.observe.on('send_message', function(url, body, data){}, responseCallback); 
```

So far the changes is working well with my use case. But please review the way I overrided the onreadystatechange and added the hook on it. I'm not too familiar with xhr methods and afraid that the way I did is not safe and would break others relying on the onreadystatechange method.
@ginotria see my comment in the pull request about what this behaviour should look like. I think this should be an optional thing. Could you make this option so that it only adds to `response_watchdog` if the `response_callback` agument is given to the function