The primary external dir is often a user-partition on the internal
storage medium. This one is safe in multi-user environments.
The "removable storage" (sdcard) on the other hand can be read by
everyone. If the former is not available or not writable, use the
latter.
closes#153
The notfification APIs now use NotificationSpec as their only parameter, which
contains all information (required and optional ones).
We no longer have separate methods and actions for SMS/EMAIL/GENERIC anymore.
The type of notification is important now, not how we received them technically.
