Feature style class

Hello, I really like your control. I have a suggestion, instead of modify directly the category style class I changed the code to use a subclass of APNumberStyle class, so if you want to create your custom style you only have to pass the subclass Class on the initialization method, similar of what apple uses with the UIPopoverBackgroundView class for change the appearance of the popover. Hope you like it.
Sorry for my bad English

![captura de pantalla de simulador ios 15 07 2014 11 10 34](https://cloud.githubusercontent.com/assets/2380325/3587578/5fde6e7c-0c3a-11e4-8cb2-cf9db763bd44.png)
![captura de pantalla de simulador ios 15 07 2014 11 10 14](https://cloud.githubusercontent.com/assets/2380325/3587582/69ceacee-0c3a-11e4-9cf1-9ae69b55524e.png)