Add parameter estimators for statistical distributions

[happyjack27]

add parameter estimators for statistical distributions

also average log-likilihood functions. given a set of observations, what is the avg log likilihood of them given the parameters of the distribution.

then, for those distributions without closed form MLE estimators (such as beta), one can use newton's method on the avg log likilihood. (maybe using method of moments to get the initial point.)

[gburtini]

I unfortunately don't have a ton of time for development on this project at the moment, but I agree with the need and would be very eager to merge pull requests that implement even part of this functionality.