[rex-remind101/MBProgressHUD] Added a drop in category with self-contain ed convenience constructors [MBProgressHUD displayGlobalHUD] and [MBProgressHUD dismissGlobalHUD]

Added a drop in category `MBProgressHUD+GlobalHUD`. It's completely self contained and respects the `UIAlertView` window level so that your `UIAlertView`s are never blocked.

All the user needs to do is call `[MBProgressHUD displayGlobalHUD]` and `[MBProgressHUD dismissGlobalHUD]` when necessary.

It uses a reference count and when the count > 0 the HUD displays, this means that someone could drop these calls in one place that uses async dispatching (such as in networking code) and never worry about the HUD displaying for too long or too short!

As long as the calls are balanced everything works out perfect.

I also added it to the Demo project :smile: