Added Warshall algorithm in Lua

Hi,

Added a new algorithm in Lua, namely [Warshall's algorithm](http://www.sroede.nl/pages/warshall/) (which is slighty different yet very close to [Floyd-Warshall](http://en.wikipedia.org/wiki/Floyd%E2%80%93Warshall_algorithm) algorithm). It does not returns the "all-pairs shortests" in a graph, but rather computes the [transitive closure](http://en.wikipedia.org/wiki/Transitive_closure#In_graph_theory) of a given adjacency matrix (set of binary relations between nodes).
Tests have also been included, with tags.

Regards,
Roland.