MDL-41101 mod_assign: introduced event base class

Also performed the following -

1) Edited existing events to use the new base class.
2) Renamed event_test.php to events_test.php to match the rest of core.
3) Moved the event tests from locallib_test.php to events_test.php.
MDL-41101 mod_assign: replaced 'view all' add_to_log call with an event
MDL-41101 mod_assign: replaced 'view submission' add_to_log call with an event
MDL-41101 mod_assign: replaced 'view feedback' add_to_log call with an event