Here's a modification to PageRank which does personalized PageRank.  The approach is basically similar to that outlined by Bahmani et al. from 2010 (http://arxiv.org/pdf/1006.2880.pdf).

I'm sure this needs tuning up or other considerations, so let me know how I can improve this.

Author: Dan McClary <dan.mcclary@gmail.com>
Author: dwmclary <dan.mcclary@gmail.com>

Closes #4774 from dwmclary/SPARK-5854-Personalized-PageRank and squashes the following commits:

8b907db [dwmclary] fixed scalastyle errors in PageRankSuite
2c20e5d [dwmclary] merged with upstream master
d6cebac [dwmclary] updated as per style requests
7d00c23 [Dan McClary] fixed line overrun in personalizedVertexPageRank
d711677 [Dan McClary] updated vertexProgram to restore binary compatibility for inner method
bb8d507 [Dan McClary] Merge branch 'master' of https://github.com/apache/spark into SPARK-5854-Personalized-PageRank
fba0edd [Dan McClary] fixed silly mistakes
de51be2 [Dan McClary] cleaned up whitespace between comments and methods
0c30d0c [Dan McClary] updated to maintain binary compatibility
aaf0b4b [Dan McClary] Merge branch 'master' of https://github.com/apache/spark into SPARK-5854-Personalized-PageRank
76773f6 [Dan McClary] Merge branch 'master' of https://github.com/apache/spark into SPARK-5854-Personalized-PageRank
44ada8e [Dan McClary] updated tolerance on chain PPR
1ffed95 [Dan McClary] updated tolerance on chain PPR
b67ac69 [Dan McClary] updated tolerance on chain PPR
a560942 [Dan McClary] rolled PPR into pregel code for PageRank
6dc2c29 [Dan McClary] initial implementation of personalized page rank