Add check for existing usernames during registration

A message will be sent from the server to indicate to the user
that the username is already taken.  It will be displayed in the div
with the id "divRegistrationStatus".
Allow server admins to accept pending registrations

South was added to INSTALLED_APPS.

The default User model was extended to add a boolean "isAccepted" field by
connecting the custom Author model in a one-to-one relationship.  This field
is false by default.

An InlineModelAdmin was defined to show the field in the Users admin page.