1. Modified Graph class to track and  return array of Edges, which will be used by
   Kruskal Algorithm for finding minimal spanning trees in a graph.
2. Modified Edge class to implement Comparable interface
3. Added DisjointedSet class to be used in Kruskal Algorim.
4. Defined MST abstract class to contain common fields/methods for bother Prim and
   Kruskal Algorithms, and public access methods.
5. Created Prim class to implement Prim Algorithm.
6. Created Kruskal class to implement Kruskal Algorithm.
7. Created TestMST class to test Prim and Kruskal algorithms.