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.
Merge pull request #15 from malept/firefox-mobile

Fix usage on recent versions of Firefox Mobile