Make autopagerize work in private browsing mode under FFx20+

Private browsing mode was changed under FFx20 to give per-window private browsing. This was only fully supported under SDK 1.14 and requires extensions to opt-in to see private windows, per:

http://blog.mozilla.org/addons/2013/02/26/per-window-private-browsing-and-the-add-on-sdk/

Also some code cleanup as editor kept complaining about missing trailing semi-colons, etc. To cherry pick just the fixes pull

https://github.com/colinmkeith/autopagerize_for_firefox/commit/9e291b5128f548764b244caf9476846a88f25f6a
https://github.com/colinmkeith/autopagerize_for_firefox/commit/f62183d95d29630344cf40c57712f726538789ba