Eloquent Refactor: Navigation #2330

Eloquent Refactor: Navigation #2330

I left the navigation_m  model so that is easier for you to compare with the Link and Group models. Check it out and let me know if I can improve anything.

### Issues
* When you try to delete a Navigation Group the public controller routing takes over. So haven' t been able to test that part or fix the issue.
* When trying to create a link I was getting some errors that are hopefully fixed in this PR #2494.
* I still have to implement the Eloquent Navigation model in the Pages module but when I visit `/admin/pages` I get the following error.
![Screen Shot 2013-02-26 at 5 33 26 PM](https://f.cloud.github.com/assets/558767/198733/994bc82a-8064-11e2-864f-d93842b8cf47.png)