Add rest of multivariate distribution tests

I've added multivariate distribution tests (#314) as well as switched the dist tests to directly comparing their results to scipy. 

These tests are faster and more directly test what we actually care about, the correctness of the logpdf. 

I am testing the other multivariate distributions by computing the pdf in numpy directly and comparing the results. It would be nice if there were a better way. Thoughts?

It would be cool to use the scipy distributions to compare for the moments and other statistics as well.