Update Singleton.php

Hi! I'm sure current realization of singleton is classic variance of the pattern. But not really correct (more then is mistaken for me) in relation to real codding on PHP projects. ;)

```php
class A extends Singleton // Singleton which used static inside of getInstance() method  
{}

class B extends A
{}

$a = A::getInstance();
$b = B::getInstance();
$c = Singleton::getInstance();

$a === $b && $b === $c; // returned FALSE
```
when u would be utilized Singleton updated then expression above returned TRUE.
Merge pull request #35 from olekhy/patch-1

use static class property instead of static variable for Singleton