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.
@btracey, I suspect stat.Mean would be pretty slow because it would have to use View to generate a slice.

@kortschak, I tried out the performance of creating the means that way and its performance is not as good as the matrix multiplication version.  I'll put together numbers when the rest of the code settles down a bit.