Add TIMEOUT to API flags

Request to add TIMEOUT option to the API for notifications.

With the TIMEOUT flag, you could send a notification to a phone and have it auto-dismiss after X seconds.  Ideally would accept argument between 1 and 86400 (1 day).

Use case is a message-of-the-day notification to all devices, with status of important things (storage space available, calendar summary, weather forecast, what-have-you data only valid for X seconds).  Right now if a device is left on a charger for a few days the MOTDs stack up and still sit there, even though the information is useless since it's old.  With TIMEOUT, you'd be able to specify that the notification auto destructs after X seconds.