make handler classes public at module level

as of rapidsms 0.15.0, handlers must be explicitly
enumerated in settings.RAPIDSMS_HANDLERS
and this makes it slightly less cumbersome to do so:

so instead of:
    RAPIDSMS_HANDLERS = ( 
        'appointments.handlers.confirm.ConfirmHandler',                             
        'appointments.handlers.move.MoveHandler',                                   
        'appointments.handlers.new.NewHandler',                                     
        'appointments.handlers.quit.QuitHandler',                                   
        'appointments.handlers.status.StatusHandler'
    )

this will work:
    RAPIDSMS_HANDLERS = ( 
        'appointments.handlers.ConfirmHandler',                             
        'appointments.handlers.MoveHandler',                                   
        'appointments.handlers.NewHandler',                                     
        'appointments.handlers.QuitHandler',                                   
        'appointments.handlers.StatusHandler'
    )