Initial Triangular distribution unit tests

I created a unit test file for the Triangular distribution (as discussed in #153).

There are still more tests to bring over from BetaTests.cs (which I used as a template), but I don't know if/when I'll get back to this, so you might as well have what I've done so far.

One word of warning though: I fixed a couple of issues that were highlighted by the unit tests, but two of the tests are still failing (due to lack of input parameter checking in the static methods, I think).