Refactor to support multivariate distributions

Here's the first cut of supporting multivariate distributions.  Included is an implementation of an uncorrelated multivariate normal distribution.  There are a few things missing still such as an approximation of the CDF which doesn't have a closed form solution, but the density and logdensity functions should be working.  I don't know if I'll have time to work on this for awhile, so I'm submitting the pull request to at least get other people looking at it.