A few improvements and fixes

Hi there,

I was using your code and noticed an opportunity for a few improvements. Here's a list of things I changed/added in this pull request:

- renamed addObserverForGroupId: observer: to setDelegateForGroupId:delegate: and typed the delegate parameter to id<RadioButtonDelegate>. The new name has a few advantages over the previous version: it indicates that there can be only one callback target for a radio button group and clarifies the type of the callback object.
- added a select method on RadioButton since you may want to programatically select a radio button (like if you need to provide a default)
- fixed a leak in RadioButtonViewController

Cheers,
Mihai