Added show command, bug fix, refactoring

A bug fix and some refactoring to prepare for adding a parallel export mode. Also added osxphotos show command.

Added

• Implemented show command: osxphotos show UUID_OR_NAME finds the photo/album/folder in Photos and spotlights it in the Photos app (#964)
• --uuid-from-file can now read from stdin if the filename is - (#965)

Changed

• Added lock files to export to minimize name collisions. This will help with implementing a parallel/multi-process export mode later.

Fixed

• Fixed osxphotos timewarp bug if timezone was null in database (#976)

Contributors To This Release

• @RhetTbull @RhetTbull
• @aa599 @aa599 for reporting the timezone bug and suggesting change to --uuid-from-file