DenseMatrix::evd() patch from David Knezevic.

This patch is from June 2011, it unfortunately got lost in my email
around that time.  Its functionality is now more readily-available
from Eigen, but I think it could still be useful in some situations.
Pass DenseVector storage directly to Lapack.
Merge branch 'dense_evd'

I compared eigenvalues computed with the new method to Octave for some
random test matrices and the results agreed.