Confirmation message after saving messages

I just noticed that after saving the admin form, no confirmation message is given to the user (at first I thought something went wrong, so I digged around in the code a little). Again, really minor issue, but thought I'd help out.

I've attached what is pretty much a one-line patch, adding the default confirmation message to ais_admin_settings_submit.

Alternatively, I suppose the form could be wrapped in system_settings_form, but that would probably mean the way (or rather, the naming with wich) the settings are stored would change, which would quite likely mean a large portion of files would have to be rewritten - which seems somewhat overkill for only a confirmation message.