virtual-homekey systemd service and multi-button support

# Systemd service
* I added a systemd service description file on the project. It is required to be run automatically on boot. I also made an install script.

# Multi-button support
* This version also includes multi-button support. It is implemented by not me, but Yeonghoon Kim.

# Clean the project
* I cleared all the trivial files like backup source code file, object file, etc.
* How about add LICENSE and AUTHOR file on the project? You'd better to clearify the license.