Create Statement::addReference

You see that this:
```php
$statement->addReference( new SnakList( new PropertyNoValueSnak( 42 ) ) );
```
is much shorter than this:
```php
$statement->setReferences( new ReferenceList( new Reference( new SnakList( new PropertyNoValueSnak( 42 ) ) ) ) );
``` 
basically copied from above and adjusted to the new methods...