Add listeners in viewWillAppear

listen in viewWillAppear.m

- (void)viewWillAppear:(BOOL)animated  {
    [[Model shared] addObserver:self 
                     forKeyPath:@"travelAgencies" 
                        options:(NSKeyValueObservingOptionNew | NSKeyValueObservingOptionInitial) 
                        context:NULL];
}

- (void)viewWillDisappear:(BOOL)animated {
    [[Model shared] removeObserver:self 
                        forKeyPath:@"travelAgencies"];
}