Example/Simple implementation for [NFR] Events & Weak References

*this pull for discussion #658*

I wrote a simple implementation for [NFR] Events & Weak References which uses WeakRef php-extension. 
Some simple example of how it works/how it use.

```php

class MyFirstListener
{
    public function afterShow()
    {
        echo 'show first listener';
    }
}

class MySecondListener
{
    public function afterShow()
    {
        echo 'show second listener';
    }
}


$eventsManager = new \Phalcon\Events\Manager();

$firstListener = new MyFirstListener();
$secondListener = new MySecondListener();

$eventsManager->attach('my', $firstListener); // without weakref
$eventsManager->attach('my', new WeakRef($secondListener)); // with weakref

// fire when two listeners still exists.
$eventsManager->fire('my:afterShow', null);
/**
 * will be shown:
 * - show first listener
 * - show second listener
 */
unset($firstListener);
unset($secondListener);



// fire when two listeners was unsetted.
$eventsManager->fire('my:afterShow', null);
/**
 * will be shown only:
 * - show first listener
 */

```