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 ) ) ) ) );
``` 
Not sure what this index thing is used for. Suspect that was not in the original method. If we do not have an immediate use case, I prefer not having this in the function signature for now.