Add a method that converts a characteristic into an observable of values

[urish]

sketch for a possible implementation:

  notifications$(char: BluetoothRemoteGATTCharacteristic) {  
    char.startNotifications();
    const disconnected = Observable.fromEvent(char.service.device as any, 'gattserverdisconnected');
    return Observable.fromEvent(char as any, 'characteristicvaluechanged').takeUntil(disconnected).map(event => event.target.value as DataView);
  }

You'd still need to stop the notifications when everybody unsubscribes, but I guess this can be added later.