Add support for PageRank calculation
Add support for [http://en.wikipedia.org/wiki/PageRank PageRank] calculation of vertices:
!PageRank is a probability distribution used to represent the likelihood that a person randomly clicking on links will arrive at any particular page. !PageRank can be calculated for any-size collection of documents. It is assumed in several research papers that the distribution is evenly divided between all documents in the collection at the beginning of the computational process. The !PageRank computations require several passes, called "iterations", through the collection to adjust approximate !PageRank values to more closely reflect the theoretical true value.