Tour callbacks

First of all, great job on this library! I've been using it for a few days and ran into a small problem today. 

I needed to perform some action once a tour was ended and currently there is no way to do this without running some interval and checking if ```$tour.isActive()``` returns false.

I've added an array to hold multiple callbacks that will be fired once the tour has ended.

**Possible use cases**
- send a request to notify backend that the user should not see the tour again
- store a variable in localstorage/cookie that tracks whether a user has seen the tour yet

**How to use**
In the appropriate controller, call

```
// start the tour
$scope.startTour = $tour.start;

// register callback
$tour.onEnd(function () {
  console.log('Tour ended');
});
```

I've tried to stick to this projects coding style but if you have any comments/suggestions I'm happy to modify :)