Rework model configuration

Rework the way how models are configured, the goal is to allow
loopback-boot to automatically determine the correct order
of the model definitions to ensure base models are defined
before they are extended.

 1. The model .js file no longer creates the model, it exports
 a config function instead:

  ```js
  module.exports = function(Car, Base) {
    // Car is the model constructor
    // Base is the parent model (e.g. loopback.PersistedModel)

    Car.prototype.honk = function(duration, cb) {
      // make some noise for `duration` seconds
      cb();
    };
  };
  ```

 2. The model is created by loopback-boot from model .json file.
  The .js file must have the same base file name.

 3. The `boot()` function has a new parameter `modelSources` to
  specify the list of directories where to look for model definitions.
  The parameter defaults to `['./models']`.

As a side effect, only models configured in `models.json` and their
base clases are defined. This should keep the size of the browserified
bundle small, because unused models are not included.