issue #287

issue #287

The MultivariateNormal trait is missing as generating the log probability requires some matrix operations. I tried with the JSci but it leads to code bloat. Maybe it would be good to based all multivariate distributions on a Scala friendly matrix implementation ([breeze](https://github.com/scalanlp/breeze) for example).

Please let me know if you want me to change anything on the design, the code style or if something is missing.

Also, let me know if you have a better idea for unit tests. The one I added occasionally fail (due to their random nature, like other tests) and increase the overall test run time.