MAINT: make expected_cols class attributes.
MAINT: make concrete_loader a class attribute.
MAINT: drop sid column before creating loader.
TST: add method to fill df with event cases.
TST: move common case df into test_events.py.
TST: improve tests and fix error message assertions.
STY: fix whitespace.
DOC: update docs.
STY: fix style.
MAINT: clean up
STY: fix indentation.
MAINT: use different assertion method to check error message for python3.4 compatibility.