Bug 717062 - preferences-branch now configurable

added a preferences-branch setting to package.json that specifies addon's preferences branch.  
if not specified, fall back to previous behavior (pref 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.