Port nonlinear least-squares fitting (gsl/gsl_multifit_nlin.h).

Ported nonlinear least-square fitting from GSL. For testing, I've implemented the example from the [GSL docs](http://www.gnu.org/software/gsl/manual/html_node/Example-programs-for-Nonlinear-Least_002dSquares-Fitting.html#Example-programs-for-Nonlinear-Least_002dSquares-Fitting), and verified the output agrees with the docs.