Port multidimensional minimization from GSL (gsl/gsl_multimin.h).

I couldn't find hooks into multidimensional minimization, hence decided to port it into CythonGSL. For testing, I've implemented examples from the [GSL docs](http://www.gnu.org/software/gsl/manual/html_node/Multimin-Examples.html), and verified the output agrees with the docs. Cheers!