adding gsl multimin algorithms with derivatives

Hi Mingzhi,

I've implemented the multidimensional minimizers from gsl as described in here:

http://www.gnu.org/software/gsl/manual/html_node/Multimin-Algorithms-with-Derivatives.html

They are unit tested. The only exception is gsl_multimin_fdfminimizer_steepest_descent which is only for demos suitable anyway. I have tried to follow your existing code conventions as much as possible.

Thanks, Philip