Server to gracefully exit on CTRL+C

[forman]

Trying to implement a way to gracefully exit server NG on CTRL+C for all platforms.

Currently, on Windows, signal handlers are not called at all.

Checklist:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/source/*
• Changes documented in CHANGES.md
• AppVeyor CI passes
• Test coverage remains or increases (target 100%)