Add __main__.py

[GreenLunar]

Similarly to python-feedgen, which would allow to keep everything within a folder.

$ python gallery_get
/usr/bin/python: can't find '__main__' module in 'bin/gallery_get/'

$ python gallery_get/
/usr/bin/python: can't find '__main__' module in 'bin/gallery_get'

[GreenLunar]

Maybe not.