Performance benchmark (#37)

Hi Pankaj,

I played with the code a bit and tried benchmarking datasets from SNAP. What I did:

* ListOfEdgesGraphReader - class for reading graphs stored as list of edges (most SNAP graphs are in this format)
* Put 3 graphs from SNAP to examples (under resources/graphs). However, I think we should find a better place for this, as they are quite heavy.
* Written examples/scala/PageRankPerformanceBenchmark - for now only global PR.

There is still a bit to be done (personalized PR, comments, paths fixes), but I want to ask if the direction is good.