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 ) ) ) ) );
``` 
If you copy something bad, the at least you should clean up the copy. Generally better to first clean up the original and then copy it.