UIAppearance implementation

I've added a `UIAppearance` implementation making it possible to edit the appearance of the progress HUD the same way you'd edit the appearance of, for example, a `UIButton`.

Now you can use the following code, to make your HUD rather ugly. :tongue: 
```obj-c
[[SVProgressHUD appearance] setHudBackgroundColor:[[UIColor blueColor] colorWithAlphaComponent:0.4]];
[[SVProgressHUD appearance] setHudForegroundColor:[UIColor yellowColor]];
[[SVProgressHUD appearance] setHudFont:[UIFont fontWithName:@"MarkerFelt-Wide" size:16]];
[[SVProgressHUD appearance] setHudStatusShadowColor:[UIColor redColor]];
```

I'd love to hear your opinions and/or suggestions.