Make a continuous deployment (CD) platform to deploy code automatically

[Yang Luo]

Currently for each merged PR in: https://github.com/casbin/casnode , we need to manually deploy it:

1. git pull
2. yarn install & yarn build
3. Deploy build static files to CDN
4. go run main.go

We need to have a continuous deployment (CD) platform to do these jobs.

[casbin-bot]

@MRGUOKING

[Yang Luo]

Already done.