Users

Covers the following stories: 
* users should exist
* Users should be able to marked as admins or catalogers
*Users should be able to login by going to /login
* Handle unsuccessful user login
* Add user information to secondary header after she has logged in
* Only logged in users can see the secondary header
@latazzajones is the `admin` column supposed to be on the `users` table? It's defined here in `db/schema.rb`, but there isn't a migration that adds this column to the `users` table. If `admin` is supposed to be on the `users` table, include it in the `users` migration. Otherwise, remove it from this file.

Please create a new pull request for this update.