multiple actions for :on option with `after_commit` and `after_rollback`

This is the feature requested in #988.

It simply adds the possibility to supply multiple actions for the `:on` parameter when defining `after_commit` and `after_rollback` callbacks.

```ruby
after_commit :do_foo_bar, :on [:create, :update]
after_commit :do_bar_baz, :on [:update, :destroy]
```