Added match_filters? to Participation model, to allow easy query if particular instance matches filters.
Added additional support for Participation.previous, no longer requiring provided dataset.
Fixed issue with self.change_was value being nil for StandingEvent.revert_change.
Added StandingCalculation class, providing majority of parsing of Participation events and calculation of StandingEvent types and values.
Fixed standing_event_spec issue in 'destroy' test.  Should expect value of zero points now.
Improved syntax for standing calculation tests.