Futures version of reading ArrayBasedDirectedGraph

Review fixes

Added twitter-util dependency to use Await.result

Review fixes

FuturePool to manage futures
Use Future.when
Indentation fix

review changes

* Using ubounded thread pool and parallelism limit
* minor fixes

review fixes

Performance fix

Avoided materializing nodesWithOutEdges with nodesWithoutOutEdges

Style fix - inline
Merge pull request #72 from szymonm/FuturesInGraphLoading

Futures version of reading ArrayBasedDirectedGraph