use Counters as partial value to simplify HadoopCountersAggregator
Merge branch 'yingyi/fullstack_fix' into wbiesing/cleanup-statistics-aggregators
use pregelix CounterAggregator for vertex compute statistics
remove conflict in comments
Merge pull request #76 from uci-cbcl/wbiesing/cleanup-statistics-aggregators

cleanup statistics aggregators