Use custom serializer for aggregation messages when the data type is int/double.
Merge branch 'master' into rxin

Conflicts:
	graph/src/main/scala/org/apache/spark/graph/impl/GraphImpl.scala
Made more specialized messages.
Additional optimizations
merged and debugged
Fixing tests
Adding unit tests to reproduce error.
Merge pull request #58 from jegonzal/KryoMessages

Kryo messages
Fix the hanging bug.
Merge pull request #60 from amplab/rxin

Looks good to me.