XHUD new overload for ShowToast

Jon--can you review the change to XHUD?  I've added another overload for ShowToast.  

The reason is that BTProgressHud out of the box doesn't look very good on iOS 7 (white on white) and so I added an overload so that you can specify the mask type now.  e.g. XHUD.HUD.ShowToast("Message", XHUD.MaskType.Black).

Thanks--I'm going to submit a similar request to BTProgressHUD