Added command-line options to download specific albums/users

Hi

I've made some modifications to your script, feel free to accept this pull request if you like. Changes are:

- New command-line option -a to download a specific album, not everything
- New command-line option -u to download photos from a specific user
- Updated command line management to argparse (optparse is deprecated)

This modifications are fully backward compatible. Just ignore the new parameters and the script works exactly like before.