Make scope of block given to Database#open narrower
[REFACTORING]Filter media type of contents at first
Use monitor on adding/removing records for thread safety
Remove recored which book was removed during sleeping
Add recored of book which was added during watcher was sleeping
Extract block around file changed to a method
[REFACTORING]Use Watch#on_file_changed if possible
[REFACTORING]Extract method Watch#start_watch
Extract method Watch#log_run_parameters
Make one line
Version bump: 0.0.3 -> 0.0.4