Torbutton shold enable firefox's "Private Browsing Mode"

We need to integrate with Firefox's Private Browsing Mode. A lot of people suggest to me that we should turn Tor on whenever Private Browsing is enabled. I think this is the wrong way to go -- but we should turn on PBM when we get enabled. This is useful for us primarily because enabling PBM will tell other addons that they also need to be private.

However, doing so will require some gymnastics to preserve Torbutton features. See: https://gitweb.torproject.org/torbutton.git/blob_plain/HEAD:/website/design/FF35_AUDIT