Added Jarvi's convex hull in Lua

Hi all,
I have been recently using [convex hull](http://en.wikipedia.org/wiki/Convex_hull) determination algorithms, and I felt like this would be a great addition to this repository. I just implemented -for now- Jarvi's [gift wrapping](http://en.wikipedia.org/wiki/Gift_wrapping_algorithm) algorithm in Lua, with tests.

Regards,
Roland.