Merge pull request #66 from pozorvlak/32bit_dfs_test

Make DFS tests pass on 32-bit machines.
A* search algorithm code.

The API is based on dijkstra_shortest_path, particularly in that it
takes edge weights as a Vector{D} rather than a function or an edge
annotation.

A* relies on a priority queue, for which I used
Base.Collections.PriorityQueue. The enqueue! and dequeue! methods
conflicted with those of Queue, breaking the BFS tests. Hence I've
wrapped the A* code in a module and exported a wrapper function into the
Graphs namespace.
A*: default value for heuristic

Conflicts:

	test/a_star_spath.jl
Test for A* algorithm.

It's the "tour of Romania" example from Norvig and Russell's AI book.
A* documentation