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
I wonder if we should have a separate activity for this.
If I create my own browser with no private tab support I'll have no way to opt out of those requests and the user will end up a non-private tab.