added nrelate migration script and fixed index notice

Migrated settings:
- nrelate_related_options:
- related_num_of_posts
- related_title
- related_show_post_excerpt
- related_max_age_num, related_max_age_frame
- related_thumbnail_size
- related_thumbnail

Also, do you remember the index database error after plugin activation? NO MORE!
yes, but only if the settings are present.