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.
Can't we just use one matrix here, and do the effective transpose-subtract into one matrix?

var xt mat64.Dense
xt.TCopy(xc)
for i, mean := range mu{
    rv := xt.RowView(i)
   for i := range rv{
       rv[i] -= mean
   }
} 

Saves an allocation and may be faster because you only have to range over mu once.