Chain crf

Slight cleanup of #4.
@vene If you don't beat me to it, I'll write a short test against GraphCRF or something.
Actually the chain shouldn't be undirected, should it? Maybe we should fix that. Do you have a nice example that would work for a directed chain but not an undirected?
Should there be two classes, ``UndirectedChainCRF`` and ``DirectedChainCRF``?