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.
While I was trying to create the unit-distribution tests for this pull request, I came across with these rounding functions. I'm not sure on how to deal with them since they mess with the gradients.