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.
I suspect the performance improvement for providing in place covariance would be pretty small, and any sane covariance matrix isn't going to be big enough to produce memory issues.  Weighted data is straightforward.  I'll implement both, and benchmark the difference for in place calculation.