Fix null args to resolve Issue #87

Adjusted ZMQContext::getSocket() and ZMQSocket::__construct() to accept null default arguments.

Reason:

It's very common in a subclass to call the parent::<function>() with the same arguments.

Example:
```php
class mysocket extends ZMQSocket {

    function __construct( ZMQContext $context , $type, $persistent_id = null, $on_new_socket = null ) {

        parent::__construct( $context, $type, $persistent_id, $on_new_socket );
    }
}
```