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.
We can also do two transpose copies. 
```
var xt mat64.Dense
xt.TCopy(x)
for i := 0; i < c; i++ {
    v := xt.RowView(i)
    mean := stat.Mean(v, weights)
    floats.AddConst(v, -mean)
}
var xc mat64.Dense
xc.TCopy(xt)
```
I'm not sure how much slower TCopy is than Copy (probably a lot because copy() is pretty highly optimized, and TCopy could be improved a lot for speed, one PR of which is under review), but this way doesn't do any allocations and it encourages using our own functions which is good.