Add createModelFromConfig and configureModel()

Add new API allowing developers to split the model definition and
configuration into two steps:

 1. Build models from JSON config, export them for re-use:

  ```js
  var Customer = loopback.createModelFromConfig({
    name: 'Customer',
    base: 'User',
    properties: {
      address: 'string'
    }
  });
  ```

 2. Attach existing models to a dataSource and a loopback app,
    modify certain model aspects like relations:

  ```js
  loopback.configureModel(Customer, {
    dataSource: db,
    relations: { /* ... */ }
  });
  ```

Rework `app.model` to use `loopback.configureModel` under the hood.
Here is the new usage:

```js
var Customer = require('./models').Customer;

app.model(Customer, {
  dataSource: 'db',
  relations: { /* ... */ }
});
```

In order to preserve backwards compatibility with loopback 1.x,
`app.model(name, config)` calls both `createModelFromConfig`
and `configureModel`.