Closes #86

- added an HTML form with id equal to 'guestUserForm' to file
'./guestcheckout.php'
- 'guestUserForm' now asks the user for their first name, last name,
email address, and phone number
- currently, the 'continue checkout' submit button just reloads the page
- added links to './login.php' and './newuser.php' next to the 'contine
checkout' submit button