Pb bowling

Pull request for the Bowling kata exercise.
This is my implementation of the bowling kata.  The solution works and passes all of the tests, giving anyone chance to see how I approached the problem and implemented it.
There is still some refactoring to be done, especially with the specs, but I wanted to give whoever reviews this some time to go through it.
If this is an array of integers, it would be better named `spare_scores`, same for `strike_score`. You could perhaps instead do `@spare_score = 0`, then `@spare_score += n` within your loop.