Added login functionality

- Changed the extensions of the pages to php
- Setup the database on my account, and hard coded the
username/password. This should really be moved to an external file that
is not under version control
- Added a user with username 'nathan' and password: 'password'
- The login form checks the database and either displays an error
message, or redirects to the homepage
- The username is stored in the session stoage, and displayed on the
homepage