Merge pull request #1 from axfelix/master

ported to OSX. not tested; almost definitely needs more work.
Removed the 'add_source_directory_tag' default config option; reorganized the structure of the config file and updated main script accordingly.
Added some inline comments to document Darwin/OSX-specific logic.
Updated README.md.
Added screencap of script in action on OS X.
Update README.md
Update config.cfg
Updated config.cfg.
Added .gitignore.
Some code cleanup/coding style changes.
Some code cleanup/coding style changes.
Normalized use of 'OSX'.
Moved confirmation dialog after folder_picker_dialog.destroy.
Use self.add_button() instead to get rid of 'Button:use-stock is deprecated' error on Windows.
fixed relative paths when creating bag elsewhere from source dir