Aggregator Support Addition

When you are running giraph job , you need to specify the MasterComputtaion class and Aggrgatorwriter class

giraph.masterComputeClass 	class 	DefaultMasterCompute 	Class for Master - optional
giraph.aggregatorWriterClass 	class 	TextAggregatorWriter 	AggregatorWriter class - optional

We can use the SimpleAggregatorWriter.java to write the aggrgator values to the file.