Simplified Aggregator check in _schedule_tasks
Removed refs attribute from Collector  (unused)
Aggregator now responsible for selecting the appropriate hook
Stackwise option is now extracted from **kwargs
Fixed constructor of Aggregator to make __repr__ of Collector possible
Improved __str__ and __repr__ methods of ViewRef
Collectors can now be specified through the constructor
Improved __repr__ and __str__ of Aggregator, Analysis and Collector
Renamed Aggregator to Collect and Analysis to Analyze
ViewRefs now accept a more readable string syntax in the constructor