UIAppearanceContainer support

Hi. Thanks for your library.

I had to customize the control appearance.

![Capture d e cran du Simulateur iOS 15 fe vr 2013 17 05 40](https://f.cloud.github.com/assets/54219/161282/b03150f2-7789-11e2-91a0-1fd3116189d6.png)

So I did add UIAppearanceContainer support for tintColor, strokeColor, shadowColor & highlightColor, so that I could write:

```objc
@implementation MyViewController

+ (void)initialize
{
    id refreshControlAppearance = [ODRefreshControl appearanceWhenContainedIn:self, nil];
    [refreshControlAppearance setTintColor:[UIColor colorWithWhite:0 alpha:0.25]];
    [refreshControlAppearance setStrokeColor:[UIColor clearColor]];
    [refreshControlAppearance setHighlightColor:[UIColor clearColor]];
    [refreshControlAppearance setShadowColor:[UIColor clearColor]];
}

@end
```

Let me know if you are interested.