dijkstra_shortest_paths_explicit()

Function that will return the actual nodes involved in dijkstra_shortest_paths as an array of (arrays of vertices).

```
julia> g = simple_graph(4)
julia> add_edge!(g,1,2); add_edge!(g,1,3); add_edge!(g,2,3); add_edge!(g,3,4)
julia> show_dijkstra_shortest_paths(g,2)
4-element Array{Array{Int64,N},1}:
 Int64[]
 [2]
 [2,3]
 [2,3,4]
```

Ref: https://github.com/JuliaLang/Graphs.jl/issues/139