Added ability to customize how networkActivityIndicator is managed

[georgemp]

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 :)