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

...Normal, and Uniform
There isn't an interface to match currently. There are several methods that are specific to distributions containing a logarithmic component, so if we wanted to roll our own there would be several different interfaces if we wanted to encapsulate all the methods Brendan has written.

However, as I briefly mentioned in bullet 2 of [this comment](https://github.com/gonum/stat/pull/4/files#r16953943), what is the benefit for hiding parameters in a slice instead of having single or multiple return values? The order and meaning of each is going to be different, so the client is going to have distribution-specific code to treat it, which defeats the purpose of hiding them in the slice in the first place to use the interface. My point in bringing this up is it will moot the point on a lot of these slice array allocations if it looks better to use multiple returns than slices. In my mind, the parameters for specific distributions isn't something you can generalize behind an interface. I understand a lot of gonum is built on "reuse-or-allocate-slices" but I don't want to overdo it.