Securely store oAuth token using  sdk/passwords

instead of localStorage
Update require-syntax, remove debugging code
Persistent "forever" desktop notifications FF22+

Only works if alerts.disableSlidingEffect=false (this is the default
value, visit about:config to change it)

Fixes #2
Desktop notifications 1.6.4 for Firefox

Changes:
- Use secure password storage for saving oAuth token instead of local
  storage.
- Preserve desktop notification until it's actually dismissed by the
  user, just like Chrome.

Tested with Firefox 23.0 and 27.0