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.
```
for i := 0; i < r; i++ {
    row := xc.RowView(i)
    var mean float64
    for _, v := range row {
        mean+= v
    }
    mean /= float64(r)

    for j := range row {
        row[j] -= mean
    }
}
```