Add User module to create, destroy and authenticate the user

For the time being this is just set up for a single user. The user's
email and hashed password are stored in the db/users.json file. Those
credentials are used to authenticate the user when they attempt to
login.

Eventually this will probably get moved to a database and support for
multiple users will be added. This is just something quick and easy to
get started with.
Implement /login and /logout endpoints

When a user is successfully authenticated a session gets created with
their information using hapi-auth-cookie. When the user logs out the
session gets cleared.