GIRAPH-728: Efficient matrix aggregators (herald via apresta)
GIRAPH-692
GIRAPH-692
GIRAPH-740
GIRAPH-739
GIRAPH-734
GIRAPH-735
Merge remote-tracking branch 'origin/trunk' into serializer
java writable api
Merge remote-tracking branch 'origin/trunk' into serializer

Conflicts:
	giraph-core/src/main/java/org/apache/giraph/partition/DiskBackedPartitionStore.java
GIRAPH-736: Bring back FindBugs (nitay)
Merge remote-tracking branch 'origin/trunk' into serializer

Conflicts:
	giraph-core/src/main/java/org/apache/giraph/utils/EdgeComparator.java