Redefine actions defined in the controller

From the Ember RC8 blog post

> we've standardized around using a hash called `actions`.When
> extending a class with actions defined, we'll merge the actions
> defined on the subclass or instance with those on the parent.
> We also support `_super` so you won't lose any flexibility
> with this approach.
> (...)
> we internally rename the property to `_actions` to avoid any
> conflicts.

http://emberjs.com/blog/2013/08/29/ember-1-0-rc8.html