cleaned up the code, work in progress

Returns BetweenNess Centrality for all vertices of the given graph.
BetweenNess centrality for a node is the importance of the node based on how many shortest paths in the graph pass through it. In short, how many shortest paths will suffer if the node is removed.
Pending work:
1. replace arrays with ArrayBuffer (Float32Array)
2. benchmark costliest loop using jsperf and put the results in comment