WIP - Add admin functionality

Also, instead of visiting the admin path directly, I'd test to make sure you can get there via a link on the main page:

```ruby
visit root_path
click_on "View Users"
```

That way it's really testing the user's experience (and tests to make sure there's a "View Users" link on the page).  

You'd also want to test that the "View Users" link does not appear for non-admins.