Fixes & optimizations to Searchable.

- Loading all records from the database at once + manual reordering to match OSS result.
- Added :include option to #oss_search & #find_results to eager load associations.
- Using after_commit instead of after_save for indexing documents.

Other minor things:

- Renamed #search to #oss_search (with alias) for simpler custom searches in models.
- Overall ruby syntax & functional code refactoring.