Power iteration: Extended interface to also return eigenvector if desired.

Old interface returning only eigenvalue is still available.
A more MATLAB-like interface, e.g.
 viennacl::tie(eigenvalue, eigenvector) = eig(A, tag);
would be nice, but pollutes the code with a lot of template hackery.

Resolves request by Charles Determan on the viennacl-devel list.