LogZ implementation

[sameersingh]

Currently c419e27624c23c5380beee14ff22c9e7832bac9a implements a simple brute force logZ term.

Need the following enhancements:

• [x] Brute force logZ
• [ ] Message passing logZ
• [ ] Gibbs sampling logZ?
• [ ] Better tests for logZ
• [ ] CRF tests for marginals
• [ ] test max likelihood that uses logZ