Allow setting properties after instantiation

I find it more readable to assign properties/methods after instantiation,
like so:

```
$stub = new s();
$stub->foo = 'bar';
$stub->callMe = function() {};
```

This commit allows doing just that.
Add support for PHP 5.2+

`php-stub` requires PHP 5.4 only because of arrays literal notation (`[]`).
While I like the new syntax for declaring arrays I think it's more practical
to support older versions of PHP if it doesn't harm library's functionality.
Besides, I think `array()` is not that ugly.