Bug fix in MatrixNormal density with unit test for non-square matrices

There was another bug in the MatrixNormal density computation when row and column covariance matrices have different dimensions. The implementation now matches the equation for density given on [wikipedia](http://en.wikipedia.org/wiki/Matrix_normal_distribution).

I also added a unit test to verify density for non-square matrices, I'm comparing the computed value to the value I got from Matlab.