Initial events for migrate and rollback actions

https://gist.github.com/TamiasSibiricus/1abf1acdc6e32792edad - sample files to test migrations
Any comments?
parent::execute() return null. So i do not see any reasons to put this method result into event as additional event data or return  it. But. It seems reasonable to set $input for Migration.beforeRollback and $output for Migration.afterRollback as event data. Same for Migration.beforeMigrate and Migration.afterMigrate