[organize] add an organize feature to copy, in a predefined sorted way (year/month/date), pictures from a directory to another
- add an OrganizeProcessor to operate the feature
- add a FileSystemWriter to manage the copy and directories creation
- refactor a method used by FlickrDownloader in FileSystemWriter
- improve the elastic checking in the main, executing the check only if the operation called by the CLI needs an elastic database running.
- enrich the README.md as needed