Initial commit for distribution package. Contains Exponential, Laplace, ...

...Normal, and Uniform
I do think it's good to have some way of getting the distribution parameters as a slice. In my mind, Fit and FitPrior (as methods on a type) are only for distributions for which the operation be done simply. Exponential and Gaussian are in the exponential family, and so both of those operations make sense. Laplace is not in the exponential family, so it has no FitPrior, but there is an efficient way of finding a maximum likelihood distribution, so it has Fit. Many distributions have no formula for the best fit given a set of samples. There are methods to get close, for example, running an optimization algorithm on the parameters of the distribution. If there is no way to get the parameters of a distribution in a consistent manner, it is impossible to write such a function.