Kruskal: Find minimum spanning tree using Kruskal algorithm

This class implements the Kruskal algorithm to find the minimum spanning tree that traverses a given set of nodes of a graph.

A graph is described as set of nodes and arcs connecting them. The class takes an associative array that has as indexes the letters of the starting and ending node. The respective array entry values represent the cost of traversing two nodes.

The class returns the minimum spanning arcs and the minimum cost to traverse them.