Add Kruskal's minimum spanning tree algorithm

Tests are put into minimum_spanning_tree.js, though algorithm itself lives in kruskal.js. The reason for this is that tests for any of MST algorithms (another one is Prim's) should probably be the same.

Another option is to extract some code from minimum_spanning_tree.js to test/algorithms/graph/kruskal.js and require that file from there. In this case there would be two test suites instead of a single one, but it would preserve clear one-to-one correspondence between algorithms and test files. I am not sure if this option is better.