added balance to guardians table to record outstanding balance'
Made aggregates table generic so it can support aggregation by any entity (aggregator)
e.g. teacher, student, school etc.
renamed some files
Controller action to trigger aggregattion
Merge remote-tracking branch 'upstream/master' into aggregate

Conflicts:
	db/schema.rb
Merge pull request #147 from adamarla/master

Aggregation of GradedResponse and some billing related tables introduced