Added login_do.php to handle user logins

The user login function in the new login_do.php script is incomplete. I wasn't sure how you wanted to go about password verification.
Please either comment or make changes/additions as necessary. I did not have your db for testing so I figured you could finalize and then QA.
I also inserted error handling for potential errors I believed to be relevant.
Also, cleaned up the design formatting and tags used for the index.php input form.