GIRAPH-682: LongNullHashSetEdges performance issue
GIRAPH-667: all workers suspended at 'saveVertices' when use out of core
Merge branch 'trunk' of https://git-wip-us.apache.org/repos/asf/giraph into trunk
GIRAPH-667
GIRAPH-468: Make Vertex an Interface (nitay)
GIRAPH-681: Graphviz Output Format (nitay)
GIRAPH-686: DiskBackedPartitionStore does not saveVertex after edges are loaded
Merge remote-tracking branch 'origin/trunk' into jython

Conflicts:
	giraph-core/src/main/java/org/apache/giraph/conf/ImmutableClassesGiraphConfiguration.java