DOC: Added EventManager example

Added a simple example algo that trades once per day at five minutes
after market_open to the EventManager docs.
TST: Added a unittest for EventManager

Added a unittest for the EventManager class thats covers the main test
cases.

Tests:
max_daily_hits: makes sure the event is not triggered more times per
day than the max_daily_hits setting.

skip_early_close_days: Makes sure that no events occur on an early
close day if skip_early_close_days is True.

period: Makes sure that the number of days between events is equal to
the period setting.