create micropost model.
add micropost model's validation spec.
add user_id-content index to micropost model.
create micropsts_controller which defined abstract actions.
remove signed_in_user before_action of users_controller.
modify signed_in_user helper.
Merge branch 'master' into topic/micropost/master
Merge branch 'topic/micropost/master' into topic/micropost/controller
implement create and destroy actions to microposts_controller.