Namespace preferences, high res icon, bug fixes

* Properly namespace preferences so they all begin with
  `extensions.pcookie`. Currently no attempt is made to transfer
  previous `pcookies.stripwww` preference.
* Add a default preferences file so don't need to worry about catching
  exceptions
* Made new icons as close as I could to original icons, but now they're
  svg
* Started work on choosing icon sets, mostly just move current icons to
  a `classic` subfolder
* Fix a bug where the icon was always placed in addon bar at startup
  (gh-10)