ENH: Dirichlet distribution

Implements a dirichlet distribution in the stats module.
For `poisson`, `planck`, `boltzmann`, 'hamming', etc., scipy capitalizes the name in the docs (so this would be "Dirichlet distribution") but it rarely refers to them except by their function names which are always lower case.

"True greatness is when your name is like ampere, watt, and fourier -- when it's spelled with a lower case letter." -- Richard Hamming