Added the ability to send a context with actions in components

It would be very useful to be able to supply a context with component actions:

```javascript
App.MyTree = Ember.Component.extend({
  click: function() {
    this.sendAction('didClickTreeNode', this.get('node'));
  }
});

App.CategoriesController = Ember.Controller.extend({
  didClickCategory: function(category) {
    //Do something with the node/category that was clicked
  }
});
```

```handlebars
{{! categories.hbs}}
{{my-tree didClickTreeNode=didClickCategory}}
```