Bug 1069915 - Privacy Panel

This is a new pull regrets for the Privacy Panel. I am very sorry to open another pull request instead of reopening the old one, but before I thought twice I have deleted my mold branch and had to reopen everything from the beginning. 
As what I have mentioned on your old PR, you should make your privacy as one menuItem inside `gaia/apps/settings/js/panels/root/` instead of making a separate panel. (Because you don't even have a panel !)

You should rename your `privacy_panel.js` to `privacy_panel_item` first (or any xxxx_item would be ok) and move it into `settings/js/panels/root/`.

Ok when you are done, you can follow https://github.com/mozilla-b2g/gaia/blob/master/apps/settings/js/panels/root/panel.js#L55 to create one `xxxxItem` instance inside `onInit` function. 

Because you don't have to automatically update something like other menuItem, you don't have to implement `menuItem.enabled` like what we did in `onShow` & `onHide`. But remember, because your instance would need to bind a click event on a real DOM, you should pass that DOM into your `PrivacyPanelItem` when initializing (Check the link above to see how we did).

And then, all the others are done in this patch and you don't have to worry about anything else.