Embed Node-Casbin directly into Casbin.js

[hsluoyz]

Currently, Casbin.js is a self-made authorization mechanism called Authorizer, which is much different with Casbin's enforcer.

Now we decided to remove all those stuff and let Casbin.js just be a wrapper of Node-Casbin. Node-Casbin can run both on browser and Node.js. For browser use, it has been used in Casbin online editor: https://github.com/casbin/casbin-editor

But Node-Casbin may need to be adjusted like adding Sync functions, to facilitate the use of frontend.

[Zxilly]

Even after gzip, node-casbin still need more than 400kb space, which is too large for the frontend.
If we did want to implement this, we should make a separate light-weight branch on node-casbin.

As a reference, react dom is 37.4kb, vue.js is 25.6kb. Webpack suggest frontend project bundle should be less than 256kb.