The problem: EUROPA doesn't have good heuristic to guide search. Thus, it can easily be misled and fail to solve even very simple problems in many domains (e.g., Blocksworlds)

The solution: Add heuristics to guide EUROPA to select the rigth node to expand next.

Note: We will likely do this after the BFS search algorithm (issue #152) is done. The BFS algorithm gives more flexibility than DFS to experience heuristics.