Allow calling super in subclasses of CoreObject.

* Add a few tests for `CoreObject`.
* Allow calling methods on your parent class via `this._super.someMethod()`.

Illustration:

```javascript
var fooCalled, barCalled, instance;

var Klass1 = CoreObject.extend({
  foo: function() {
    fooCalled = true;
  }
});

var Klass2 = Klass1.extend({
  foo: function() {
    this._super.foo();
  },

  bar: function() {
    barCalled = true;
    this._super.foo();
  }
});

instance = new Klass2();
instance.bar();

assert(fooCalled, 'foo called');
assert(barCalled, 'bar called');

fooCalled = false;
instance.foo();

assert(fooCalled, 'foo called');
```
Merge pull request #1721 from rwjblue/allow-calling-super

Allow calling this._super in subclasses of CoreObject.