UIAppearanceContainer support

[groue]

Hi. Thanks for your library.

I had to customize the control appearance.

So I did add UIAppearanceContainer support for tintColor, strokeColor, shadowColor & highlightColor, so that I could write:

@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.