docs.json: add lib/model.js

I noticed that `ModelBaseClass` was missing from our API docs. The class provides some useful methods like `ModelBaseClass.defineProperty(prop, params)`.


![screen shot 2014-12-02 at 8 29 36](https://cloud.githubusercontent.com/assets/1140553/5259222/61c7fab0-79fd-11e4-8563-e5d8ad2ba9f1.png)

/to @raymondfeng @crandmck please review