Remove array type-hint from Collection::createDBRef() param

See: http://php.net/manual/en/mongocollection.createdbref.php
Remove events/logging for Collection::createDBRef()

This method is incredibly trivial, and there is little value in dispatching events or logging its calls.
Remove logging for Database::createDBRef()

This is being removed for the same reason as the Collection method (creating DBRefs is a very trivial operation).
Database::getDBRef() should dispatch events

Also create unit test for Database's event dispatching.
Merge pull request #126 from doctrine/dbref

DBRef database/collection method improvements