Bayesian assignment probabilities

Hmm...this depends on the bayes_score function from the view_bayesian_results branch/pull request; I'm not sure what the best way to separate out the new changes would be, but they should all be in the final commit (f7ecdc5aa12a4447e8805e7c2f4a646a30a8e98e)

Anyway, this allows for setting unequal probabilities for different alternatives; in particular, it allows for rebalancing the probabilities based on the Bayesian calculations of the probability of each alternative being optimal, allowing for eventual convergence on the best outcome using all information gathered (and a nice property of improving total conversions over the course of the experiment and afterwards)