added alpha and beta params for beta-binomial

Added alpha and beta as parameters for beta-binomial.  It's useful in display advertising to vary this parameter due to the extremely low conversion rates (using beta 2,2 for example for something more related to bernoulli trial). 

I'm a horrible production code writer so feel free to suggest changes to my code in the pull request =)

I tested this with varying the parameters for two highly imbalanced groups (90%/10% a/b test) with 0.01% ctr rates and the parameters behave as expected.