Support CKCountdownButtonDelegate

The `countedDown` delegate will be invoked when timer count down to zero.

At first, I don't know how to write a test to verify the delegation. After a lot of search, I finally implemented.

It seems that Kiki is better than Specta, because Kiwi support Mocks and Stubs but, Specta depends on other standalone modules such as OCMock.

Here is a list of user links I found

- [Mock and Stubs in several popular lib](https://github.com/jbrunton/iosdemo/wiki/Mocks)
- [Mock and Stubs in Kiwi](https://github.com/kiwi-bdd/Kiwi/wiki/Mocks-and-Stubs)
- [Mock in Specta](https://gist.github.com/orta/d3ea1b2d10e80d757550), I cannot find `Mocta` mentioned in article.