Add support for alternative parameterization of beta and gamma

Kruschke often defines the priors for `Beta` and `Gamma` in terms of mean and sd. There's a simple transformation to move to `alpha` and `beta`. Instead of requiring to do this in model code, this PR allows to alternatively submit `mu` and `sd` to `Beta` and `Gamma` which then does the transformation under the hood.

We do something similar for `Normal` where we allow to either specify precision `tau` or standard deviation `sd`. I think this has worked out quite well and makes the model specification much easier on the eyes.