Enable client model configuration and extension

 * Expects loopback style model-config.json as fourth parameter
  * Expects `dataSource = "remote"` for REST-exposed models

 * No backward compatibility break if model-config parameter missing

 * If model-config present, any model not in the file will be skipped

 * If model-config present, any model not REST-exposed will be skipped

 * Look for model.js extension files in `modelConfig._meta.sources`

 * Expects model.js files to be nodejs modules returning same object
   as ones expected in the loopback boot process

Note : `fixture/model-config.json` contains all models used in the test
       suite so nothing will break. Only MyModel contains custom logic.
Enable local (non-REST) object models to be generated/extended
Enable dependency injection for defined client models
Fix deprecated express res.send(status, ...) to res.status(code).send(...)
Fix issue with relative path and backward compatibility