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.
They do not contain the same memory.  The ones variable is a vector of ones with the same number of elements as the number of rows in the matrix, while the mu vector is a vector of column averages with the same number of elements as the number of columns in the matrix.  The ones vector is multiplied with the matrix we're taking covariance over to find the sum of the columns, which is then scaled to the number of elements.