allow global aggregators to be specified in xml

explicitly setting the aggregator in the PregelixJob constructors would
override any values read in from the conf's resources.

Instead, this commit doesn't set the conf explicitly and instead
specifies an array of aggregator class names which will always be in
place when `getGlobalAggregatorClasses` is called.

Conflicts:
	pregelix/pregelix-
api/src/main/java/edu/uci/ics/pregelix/api/job/PregelixJob.java