selenic-zsign v0.5

Changes:

• New build script
• Code changes here & there that makes it play nicer with Windows
• Ongoing restructuring of the codebase
• Updated building instructions for Linux, macOS, and Windows.

Implemented PRs from zhlynn/zsign:

• zhlynn/zsign#301
• zhlynn/zsign#250
• Recoded version of zhlynn/zsign#273