Set any instance of UIView for both titleView and subtitleView properties

This should be similar how `UINavigationItem`'s `title` and `titleView` properties work; then `SMCalloutView`'s interface would be similar to:

``` Objective-C
@interface SMCalloutView : UIView

// ...

@property (nonatomic, copy) NSString *title, subtitle;
@property (nonatomic, retain) UIView *titleView, *subtitleView;

// ...

@end
```
