Add aggregation method for aging report, usages:
include AggUtil # in controler
@collection=... # choose invoiceItem collection
as_of_date = Date.new(2013,01,01) # choose as of date
result = aging_as_of as_of_date # get the report
## example result
[{current=>0, thirty=>0, sixty=>0, ninety=>0, oneTwenty=>0, oneTwentyMore=>110.0, currency=>USD}]