Remove lint errors

[zaerl]

1. Visibility must be declared on property $max_wxr_version
2. The var keyword must not be used to declare a property
3. Visibility must be declared on property $id
4. Visibility must be declared on property $version
5. The var keyword must not be used to declare a property
6. Visibility must be declared on property $authors
7. The var keyword must not be used to declare a property
8. Visibility must be declared on property $posts
9. The var keyword must not be used to declare a property
10. Visibility must be declared on property $terms
11. The var keyword must not be used to declare a property
12. Visibility must be declared on property $categories
13. The var keyword must not be used to declare a property
14. Visibility must be declared on property $tags
15. The var keyword must not be used to declare a property
16. Visibility must be declared on property $base_url
17. The var keyword must not be used to declare a property
18. Visibility must be declared on property $processed_authors
19. The var keyword must not be used to declare a property
20. Visibility must be declared on property $author_mapping
21. The var keyword must not be used to declare a property
22. Visibility must be declared on property $processed_terms
23. The var keyword must not be used to declare a property