MAINT: refactored add_event api_method

Changed the arguments in event_dt. The default start_dt arg is now
None and the timezone arg was deleted. When left as None, start_dt
defaults to TradingAlgorithm.sim_params.period_start.
MAINT: cleaned up EventOffset class

Changed the default start_dt argument in EventOffset
to None.

EventOffset.handle_data now returns True
if the event is triggered, this was to make testing
more convenient.
TST: Added tests for EventOffset class

Added unittests for the EventOffset class.
There are probably more tests that should
be written, but the main ones are covered here.