Reorganize so that the example project consumes the pod using a Podfile.

This is better form so you don't need to have two copies each of ```Luhn.h``` and ```Luhn.m```. All you need to do now is run ```pod install``` from the example project directory as if it were a third party app consuming it. Let me know if you have any questions.

Just wanted to make this PR first cause I wanted to add some tests for it after seeing someone use this pod. Love that you wrote the Luhn algorithm in Objective-C. :octocat: :thumbsup:

Cheers.