Update install instructions

[tresni]

• Make setup.cfg use requirements.txt for install_requires
• List winsdk in the requirements.txt to make setup easier
• Create a build script for MacOS
• Update README instructions

The setup.cfg requirements were out of date with the requirements.txt. This meant that if you built a binary using the setup.cfg, it would have an outdated version of librespot and be unable to login. Given that setuptools and pip can both use properly formatted PEP-508 syntax and onthespot is not using any pip-specific syntax, this can be unified into the requirements.txt file for ease.

[tresni]

My bad, thanks for getting that!