[WIP] Linear chain CRF

Add basic linear-chain CRF, priceless for NLP tasks.  Not sure how to test it, I'll come back to this.

Rendered less useful by lack of support for sparse matrices at the moment. I did run some successful experiments on a box with 100 gigs of memory ;)  I got accuracy only slightly under crfsuite's averaged perceptron, with the implementation in PR#3.