implemented hoodie.task events that can be subscribed to

* start
* cancel
* error
* success

Example:

```js
hoodie.task.on('message:error', function(error, message) {
  showError('Your message ' + message.subject + ' could not be sent: ' + error);
})
hoodie.task.on('message:success', function() {
  playMessageSentSound();
})
```

* * *

This week of work on @hoodiehq is sponsored by Cargo Media.

   Their slogan is “Small Team, Big Impact” and they are hiring
   a Frontend and Backend Dev: http://go.hood.ie/cargo-media