Added ability to customize how networkActivityIndicator is managed

In some cases, it might be useful to customize how the networkActivityIndicator visibility is turned off or on. For example, we might have a central manager to do this, in case there are other threads that are also accessing the network. In my app, I use AFNetworking which manages a count of networkActivityIndicator setVisible requests and turns it on and off appropriately. Directly setting the activity indicator visibility to on/off via UIApplication in this case will not be truly representative of the actual network activity. So, I set up a block property where this custom behavior can be defined

    modalWebViewController.customSetNetworkActivityIndicatorVisible = ^(BOOL networkActivityIndicatorVisible) {
            if (networkActivityIndicatorVisible)
                [[AFNetworkActivityIndicatorManager sharedManager] incrementActivityCount];
            else
                [[AFNetworkActivityIndicatorManager sharedManager] decrementActivityCount];
        };

Hope others will find this useful as well :)