Implement skeleton of sign-up and log-in features

To allow customizations and progress tracking, users are now asked to create their own accounts, where their progress can be stored.

- Modified index.php to include very basic (skeleton) signup features. The login screen is displayed when no user is logged in; the main screen is displayed when a user is logged in. The login screen and the registration screen are not styled at the moment.
- Added base.php, register.php, and logout.php, which are used to start sessions, register, and logout, respectively.
- Wrote separate stylesheets for main, login and registration screens.
- Updated README to account for new signup feature.