Only load models if modelsPath is set

My models are in subdirectories like this:
```
api/user
  user.controller.js
  user.model.js
api/song
  song.controller.js
  song.model.js
```
I load the models using a custom function and then create `penguin.Admin` with this:
```
  var admin = new penguin.Admin({
    modelsPath: null,
  });
```
Maybe a better way to do it would be to add a boolean.