Provide implementation for Visitor

[BartVandewoestyne]

Apparently, there is no sample code for the Visitor pattern. I don't know if this was intentional, if it was due to the lack of sample code in the book, or if I just forgot and moved on with my life.

We should check whether the book provides enough info to provide sample code that compiles and runs. If not, we should mention why no sample code is given.