allowing the ability to show joke on screen

While this does not show the random image, it is getting a step closer by showing a joke from the API.

had to call the function on the main thread in order to
reflect the changes in the UI.

https://github.com/GRCocoaHeads/ChuckNorrisIOS/issues/29

I think that this pull request closes this issue as well:
https://github.com/GRCocoaHeads/ChuckNorrisIOS/issues/14
Use a "let" here. That will give a warning if the jokeController variable gets reassigned.