Add wiener distribution

Code from by Joachim Vandekerckhove http://www.cidlab.com/supp.php?o=wienerstan, which was created for stan v1.3; tweaked by Mike Lawrence to (hopefully) work for 2.1.0.
Is the problem that you get the wrong derivatives compared
to finite differences?

If so, this will need to be rewritten so that autodiff can
work or the gradients will have to be figured out analytically.
Obviously the analytic solution is more efficient and preferred
if it's possible.