durandal.js: added interface to deffered object

Added interface for deferred object, returned from event subscription.

```
export var on: (events: string, callback: Function, context?) => IEventSubscription;

interface IEventSubscription
{
    /**
      * This function adding callback to event subscription
      */
    then(thenCallback: any): void;

    /**
      * This function removing current subscription from event handlers
      */
     off(): void;
 }
```