Allow to have _super in complete

Example:
http://jsfiddle.net/rj7c3/
```
var e = Ractive.extend({
    init: function() {
      console.log('parent init2');  
    },
    complete: function() {
     console.log('parent2');   
    }
})

var ex = e.extend({
    init: function() {
        this._super();
      console.log('parent init');  
    },
    complete: function() {
        this._super();
     console.log('parent');   
    }
})

var ractive = ex.extend({
            el: 'container',
            template: '#myTemplate',
            data: { greeting: 'Hello', recipient: 'world' },
    complete: function() {
        this._super();
     console.log('child');   
    }
        });

a = new ractive();
```
Output:
```
parent2
parent
child
parent init2
parent init 
```
Please review before accepting pull request. It might break other things.