reduce cost of row operations in QR
This is commit  61aa81a28fef7a23002b2c3fd9ed2bc2f5b508c9 to
github.com/gonum/matrix by kortschak on Jan 15, 2014. It's now hard to
merge directly.
Reproduce 4 commits to github.com/gonum/matrix by kortschak and
dane-uultd on Jan 14, 2014. It's now hard to merge.

 Fix svd for wide matrices by operating on transposed copy.

 Update the comments describing SVD behaviour and ensure that the
 correct matrices are constructed.

 Output disagrees with R's svd function in sign only.
swap(); save a row copy in pivotRows()