MAINT: Rework event datasets.

- Refactored EventsLoader and BlazeEventsLoader to not require a
  subclass per dataset.  Instead, you now pass a map from columns to
  event fields directly to the EventsLoader constructor.

- Removed a large number of Quantopian-specific datasets and associated
  tests.

- Rewrote the core logic of EventsLoader and BlazeEventsLoader to share
  index calculations across multiple requested columns.

- Fixed a bug where event fields were incorrectly forward-filled when
  null values were present in an event.

zipline/testing/__init__.py

@@ -23,7 +23,6 @@ from .core import (  # noqa
     empty_asset_finder,
     empty_assets_db,
     empty_trading_env,
-    gen_calendars,
     make_test_handler,
     make_trade_data_for_asset_info,
     parameter_space,