Provide GraphX interface

For open source adoption, it could be nice to have a drop-in replacement for the [GraphX PageRank](https://spark.apache.org/docs/1.4.0/api/java/org/apache/spark/graphx/lib/PageRank.html#run(org.apache.spark.graphx.Graph,%20int,%20double,%20scala.reflect.ClassTag,%20scala.reflect.ClassTag)).