Add checked-out item functions to loan

- Added .checked_out_title & .checked_out_title?
- Added .checked_out_author & .checked_out_author?
- Fine-tuned all .checked_out_* functions for faster selection
- Updated CHANGELOG
Add checked-out item selectors to loan

- Added .select_checked_out_barcode
- Added .select_checked_out_title
- Added .select_checked_out_author
- Updated CHANGELOG