SSSP Bellman-Ford implementation (WIP)

A simple SSSP implementation using Bellman-Ford Algorithm. Few things still to be done:
1. SSSP parent tree construction
2. Verification function which is similar existing one for BFS kernel (to application/demos/bfs), but has some additional checks:

1) the SSSP tree is a rooted tree and does not contain cycles,
2) a node and its parent are joined by an edge of the original graph,
3) w(u, v) + d(u) - d(v) ≤ 0 for all unordered input edges {u, v} where d(u) < d(v), and
4) w(u, v) + d(u) - d(v) == 0 when u is a parent of v.
http://www.cc.gatech.edu/~jriedy/tmp/graph500/