Add Casbin model and policy management

[hsluoyz]

See: #37

We once developed two similar systems: Casbin-Dashboard and Casbin-Cloud, but both projects failed finally. You can just grap some inspirations from them:

Casbin-Dashboard

1. Source code: https://github.com/casbin/casbin-dashboard
2. Online demo: https://dashboard.casbin.org/

Casbin-Cloud

1. Source code: https://github.com/casbin/openstack-policy-editor
2. Online demo: https://cloud.casbin.org/

However, don't copy them, because they have failed and I don't think their designs are good. Just use them as reference and starting point to understand this area.

[WindSpiritSR]

Can I work on it?