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.
This code doesn't seem to take gradients into consideration at all, neither via operands_and_partials, nor via auto-diff (because it uses value_of instead of keeping vars).