Add encryption support to the API

Hi!

I think encryption should be supported by the API. It's easy to see the encryption method used by looking into the Chrome extension code, so clients which use the API can easily reproduce it. Security would be very much improved. Maybe a simple "encrypted=true" flag in the URL?

EDIT: I was able to make encryption work for notification text, but not for the notification title? Maybe I'm missing something? I think it should be documented.

Thanks!