SPFA algorithm added

[shuding]

Add SPFA algorithm and tests :P

From Wikipedia:

Shortest Path Faster Algorithm (SPFA) is an improvement of the Bellman–Ford algorithm which computes single-source shortest paths in a weighted directed graph.

procedure Shortest-Path-Faster-Algorithm(G, s)
1    for each vertex v ≠ s in V(G)
2        d(v) := ∞
3    d(s) := 0
4    offer s into Q
5    while Q is not empty
6        u := poll Q
7        for each edge (u, v) in E(G)
8            if d(u) + w(u, v) < d(v) then
9                d(v) := d(u) + w(u, v)
10                if v is not in Q then
11                    offer v into Q

[coveralls]

Coverage remained the same when pulling ba02bd0 on quietshu:master into c660898 on felipernb:master.

[coveralls]

Coverage decreased (-0.19%) when pulling 4b3b8f7 on quietshu:master into c660898 on felipernb:master.

[felipernb]

Thanks! 🍺