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.
Since you are taking a Dense copy of x already, you can get what you are doing here more cheaply by iterating over the rows and then iterating over the view on each row.

Untested:

```
xc := DenseCopyOf(x)
mu := make([]float64, r)
for i := 0; i < r; i++ {
    for _, v := range xc.RowView(i) {
        mu[i] += v
    }
    mu[i] /= float64(r)
}
```