Create Markov chain example

[mschauer]

@cscherrer (perhaps @trappmartin)

[cscherrer]

For MeasureTheory, we need an instance for rand and one for logdensity. I think rand can just return a Sampled. I'm not sure yet about logdensity.

Also, after making this I realized there may be better ways to do this using existing combinators. But I need to understand those better before I can do much with this.