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.
No, I haven't tried finite diff tests yet because I came across the problem above (of line 89). This one here was more a theoretical concern (about discontinuous gradients).

In fact, I was hopping the code was ok [and didn't check it] when I started writing the tests, but it seems now that it isn't and I might not know enough about Wiener process to fix it.