Add a helpful warning

Because almost every property on a class definition is a function, it's
easy to accidentally say this out of habit:

````javascript

App.MyController = Ember.Controller.extend({
  actions: function() {
    doThing: function() { ... }
  }
});

````

When you meant to say this:

````javascript

App.MyController = Ember.Controller.extend({
  actions: {
    doThing: function() { ... }
  }
});

````

This doesn't product any errors or warning, it just makes your handlers
invisible. My change adds an assertion.
Merge pull request #3749 from ef4/actions-warning

Actions warning