2.0 nav

Addresses #358:

- [x] Enhance CroogoNav so that it can store multiple menu, eg:

```
  CroogoNav::setActiveMenu('sidebar');
  CroogoNav::add(....)
  CroogoNav::add(....)

  CroogoNav::setActiveMenu('top');
  CroogoNav::add(....)
  CroogoNav::add(....)

  // or
  CroogoNav::add('top', 'users', array(....));
```
TODO:

- [x] Decide on how would we display the menu, eg: using MenuHelper::menu() ?