model.js - added a typeKey property

Returns the typeKey of the model, for example if evaluated on an instance of App.List, it would return 'list'
    
The property is useful because it allows you to get the typeKey from within handlebars directly from the model
  
Usage case: generic templates
```handlebars
{{#link-to item.typeKey item}}My type generic link!{{/link-to}}
```