#43 skip user creation page if user already exists

[Katie Bechtold]

Upon installation, the administrator is prompted to
create a new user with a login id, e-mail address, and
password. After that's entered, the appropriate tables
are created in the database, and included in that is a
check as to whether a users table already exists, and
if it does, whether it has at least one user. If
there's already at least one user, install.php does not
add the new one.

install.php should really check whether a new user
needs to be created *before* the administrator enters
the new user info.