Add autoresizing

I needed the circular progress to resize (e.g. rotating from landscape to portrait). It seemed much cleaner to use `frame` as the definitive size, instead of `radius` and the center point. (`radius` is now a computed property.) I also reworked the initializer, so that the view can be created with a simple `initWithFrame:`. (I kept backwards compatibility so as not to break others code.)

I also added a few more modern obj-c idioms. `instancetype` and `NS_ENUM`.

The example app is updated, so you can test that everything works. (tap to resize)