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'm not sure that we need a comment about BLAS here. Most code using Dense will need to register a blas function, and that number will only increase as the matrix and blas packages improve. I don't think we want this comment on every function in Gonum. It's clear that this uses Dense from the return signature. @kortschak ?

The "the matrix returned ... " comment would be better in the first paragraph.

"The weights, wts, should have length equal to the rows in" 

One space before cov.

"cov should either be ... or nil in which case a new ..."