Provide pkg uninstaller for macOS

[x1ddos]

It appears to be hard to manually uninstall a macOS .pkg:
https://wincent.com/wiki/Uninstalling_packages_%28.pkg_files%29_on_Mac_OS_X

Anyone attempting to uninstall bitbox-bridge on macOS will be having a hard time.

Unless I missed it and there's an easy way to uninstall?

[benma]

@NickeZ maybe you know?

[NickeZ]

I don't know from the top of my head, but I can investigate a little bit