Minor logging changes.
Added kcore implementation.

Conflicts:
	graph/src/main/scala/org/apache/spark/graph/Pregel.scala
Fixed results of cherry-pick
Kcore debugging.
Changed kcore filenames.
Fixed kcore. now works.
Added more logging.

Conflicts:
	graphx/src/main/scala/org/apache/spark/graphx/lib/Analytics.scala
Merge pull request #3 from dcrankshaw/osdi_with_kcore_for_merge

Osdi with kcore for merge