Fix ode4 argument order in comments

If you're like me, you spend waaay too long wondering why the simple RK4 solver isn't working instead of actually checking the function itself. The comments above the function had the argument order wrong - this tiny commit fixes that.
Merge pull request #48 from swt30/fix-ode4-comments

Fix ode4 argument order in comments
Make fdjacobian available to all solvers.
Add solver ode23s for stiff systems (based on modified Rosenbrock triple).
Updated README.md and added some comments to ode23s.
Rebase.