Bug 717062 - root pref branch now configurable

added a prefsRoot setting to package.json that specifies the root preferences branch.  
if not specified, fall back to previous behavior (root branch is extensions.jetpackID).
modified the cfx (xpi builder) and some packages (like simple-prefs).
adapted some cfx/package tests and some test addons,
plus added a couple of new ones. 
all tests pass.