Derivative algorithms

Hi all,

Submitting an algorithm to evaluate the [derivative](http://en.wikipedia.org/wiki/Derivative#Rigorous_definition) of a given function at some point using differential approximation (based on Newton slopes equations).
I am widely using such algorithms in my daily work, so I thought it would be a nice addendum to this project.
Tests have also been included, aswell.

Note: I also brought some fixes/ehancements in the previous Jarvis March algorithm.

Regards,
Roland.