Implement the simple version of pregel module on REEF.

[JunhoeKim]

So far, Elastic-table has been developed to be well adapted to ML application.

However, elastic-table can be applied to not only ML app, but graph processing app. To prove this, we will introduce the Pregel(a system for large-scale graph processing) system and run on ET.
Before doing this, implementing the simple version of pregel without ET can be good to be a previous step.