Raindrops: Updated to use functions. Example now uses anonymous function

I decided to structure the exercise in a similar manner to the python implementation. I also decided to use an anonymous function in the example to show how to use them.

I'm starting to get more comfortable with branches.
In the previous implementation we had three functions that were essentially doing the same thing. We only changed one value between the ping, plang, and plong functions. The values I need are all determined from $num, can be calculated in one line, and I'm probably not going to need any those three methods again. Within the confines of the exercise it was a one time one place solution. This is why I decided to use an anonymous function.

When I go to refactor the function then my one time one place helper method is right there in the same scope, and isn't polluting the namespace. They are also really helpful for methods with callback functions.