CovarianceMatrix function

As discussed previously in #15, this implements a two-pass covariance calculation for mat64.Matrix.  I'm wondering if we should require BLAS to calculate covariance or if I should write an implementation with and without it.  Also, with better knowledge of the mat64 package I believe some of the memory allocations can be eliminated.

Currently a WIP, this pull request isn't anywhere near ready for merge.  Also, the .travis.yml file will have to be updated to get the BLAS dependency.  PTAL @kortschak, @btracey, I could use your advice.
Sorry, weights should have been wts. The code needs to compute the weighted mean.

I think it's the same number of allocations as the unrolled version. They both allocate the two matrices xt and xc, and nothing else. I'm happy to not use AddConst, but Mean is already correct for computing weighted means of data slices, which is exactly what we want here; the weighted mean of each column.