Provide implementation for the State pattern

[BartVandewoestyne]

Apparently, there is no sample code for the State 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.

[grantrostig]

There is code in Gamma. I may be able to figure it out eventually and submit is.

[BartVandewoestyne]

Go for it! :-)