Changed in read for pull#7

@John: A lot changed after you sent the pull request so resolving conflicts was hard. I have done the changed which you have suggested here. 

Please have a look. Thanks.
Backlinks data is sent to the *PageRankService* over HTTP (e.g. by a web crawler as it processes web pages). The service persists the data into a *backLinks* dataset upon receiving it. The PageRank for known pages is computed periodically by a *PageRankProgram*. The program uses the *backLinks* dataset as an input and persists the results in the *pageRanks* dataset.