User creation and login

First off, this branch removes `rack-ssl-enforcer` in favor of using Rails' built in methods of enforcing https.

Secondly, this branch adds the ability to create users and log in users.

A user can be created by `POST`ing to `/users` with post parameters `username`, `password`, and `password_confirmation`.

A user may be logged in by `POST`ing to `/session` with post parameters `username` and `password`.

Both of these will return a session cookie which must be stored and passed on to subsequent requests to the API.

I left in the `show` and `index` routes on `/users` for debugging, but if anyone thinks that it would be best to remove them and only use the console, I'll remove them.