Bug 1081731 - Simple support for prviate browsing +autoland

This is an experiment which creates two entry points to private browsing on FirefoxOS. One being in the context menu when long tapping on any link, and the other being in the browser "overflow" menu at the top of every page.

https://bugzilla.mozilla.org/show_bug.cgi?id=1081731
Good point. Can apps override this view activity today? If the app has no way of making what they are doing "private", then we should probably not allow the activity to go to those apps.