[FEATURE conditional-observers-and-listeners]

I find myself doing the following a lot:

```js
SomeClass = Ember.Object.extend({

  enabled: true,

  value: 'foo',

  valueDidChange: function() {
    if (this.get('enabled')) {
      // do something after value changes only when enabled is true
    }
  }.observes('enabled', 'value')
});
```

That works fine under normal circumstances but what happens when your dealing with a large number of these objects and `value` changes frequently when `enabled` is `false`? The `valueDidChange` observer fires needlessly on all of those objects, basically it adds additional overhead that accomplishes nothing.

If we refactor the code a bit, we can avoid this:

```js
SomeClass = Ember.Object.extend({

  enabled: true,

  enabledDidChange: function() {
    if (this.get('enabled')) {
      Ember.addObserver(this, 'value', null, 'valueDidChangeWhenEnabled');
    } else {
      Ember.removeObserver(this, 'value', null, 'valueDidChangeWhenEnabled');
    }
  }.observes('enabled'),

  value: 'foo',

  valueDidChangeWhenEnabled: function() {
    // do something after value changes only when enabled is true
  }
});
```

Now the `value` observer will only fire when `enabled` is `true` because it only exists when `enabled` is `true`. Basically, we've introduced a conditional observer. There were quite a few places where I found this sort of thing useful. So I came up with this commit that essentially allows one to do the same thing in a bit more declarative way:

```js
SomeClass = Ember.Object.extend({

  enabled: true,

  value: 'foo',

  doSomething: function() {
    // do something after value changes only when enabled is true
  }.observes('value').when('enabled')
});
```

This commit also allows `when` to work with listeners:

```javascript
Ember.TextField.extend({

  selectIsEnabled: true,

  select: function() {
    // Executes on "focusIn" only when selectIsEnabled is true
    this.$().select();
  }.on('focusIn').when('selectIsEnabled')
});
```

Thoughts?
Should just be `ember-metal/mixin` (without the extension).