Fix usage on recent versions of Firefox Mobile

Firefox Mobile has included support for the unprefixed version of the web
notifications API since version 22. Unfortunately, it also seems to have kept
`navigator.mozNotification` around, though it no longer seems to work. Move
conditionals around in `permissionLevel()` so the unprefixed check takes
precedence over the `moz`-prefixed check.

Fixes GH ttsvetko/HTML5-Desktop-Notifications#12.