Basic admin area

There's a few options here:

- [rails_admin](https://github.com/sferik/rails_admin)
- [ActiveAdmin](http://activeadmin.info/)
- Custom solution with [Foundation](http://foundation.zurb.com/)

Here are the steps I need to take:

- [ ] Install one of the above solutions
- [ ] Create `Admin` app that goes on top of Noted
- [ ] Designate user levels (could be as simple as an `admin` field on `User`)