Add an option to reject a message

Currently we can only respond to a message, remotely resolving a promise awaiting in the other frame. We'll find it useful if we could also reject a message like this:

``` javascript
talker.onMessage = function(message) {
  message.reject(reason);
};
```

Then, the `reason` string will be send to a talker in other window and the awaiting promise will be rejected with the `reason`.
