bug 717062 - preferences-branch now configurable

added preferences-branch (package.json), modified cfx (xpi builder),
packages (simple-prefs), adapted tests (cfx, packages, addons), added
new ones.  all tests pass..
bug 717062 - preferences-branch, moar tests

added tests for curly braces ID and invalid preferences-branch value
bug 717062 - preferences-branch, even more..

..tests
Bug 717062 adding a test for simple prefs where a predefined id with an @ symbol is used
Bug 717062 - Root preferences branch for simple-prefs needs to be configurable r=@erikvold

Merge branch '717062'