bin/ppost.rb: Fix typo.
lib/photo_registrar.rb: `add_tags' method is new.
bin/ppost.rb: --add-tags option.
lib/model/photo.rb: Photo#add_tabs returns array of added tags.
lib/photo_registrar.rb: PhotoRegistrar#add_tags returns added tags.
lib/photo_registrar.rb: Fix PhotoRegistrar#add_tags.
bin/ppost.rb: Fix message.
bin/ppost.rb: Fix typo in help message.