Modularize nndownload

[jaeseopark]

Converted nndownload into a python module.

The standalone mode continues to work as it currently does. no changes there.

It's probably very hard to see what parts of the code changed, because diff in Git is all red and green. Here is a better view of what has changed exactly.

summary:

1. changed how logger is initialized and used. This is to allow nndownload to inherit any log configurations from the caller-module.
2. main() no longer calls sys.exit() when an exception is thrown. Instead, it simply bubbles the exception up, so the except block of the caller-module can do the rest.

I put the pypi badge in the README but haven't actually uploaded the package to pypi. It's on TestPyPi, though.

[AlexAplin]

• Logging and quiet flags no longer controlled by the module. These should always be available, so I've changed how the logger behaves.
• Moved __main__ back into the main script.
• Changed module function to execute() since it's basically a wrapper.

[AlexAplin]

@jaeseopark Please add me as the owner of the package (https://pypi.org/user/AlexAplin/).

[jaeseopark]

Thanks for the reminder. I added you.

Do you know how to transfer the ownership entirely? I never intended to be an owner of this project.

[AlexAplin]

Thanks, I went ahead and removed you.