Revert "Revert "Bug 717062 - Root preferences branch for simple-prefs needs to be configurable r=@erikvold""

This reverts commit 3784678ed07b074295bacd7d67ca259b043c0b8c.
Bug 717062 exposing require("sdk/self").preferencesBranch
Merge branch 'master' of github.com:mozilla/addon-sdk into 717062v2
Bug 717062 Adding regression test
Merge branch 'master' of github.com:mozilla/addon-sdk into 717062v2

Conflicts:
	lib/sdk/simple-prefs.js
Merge pull request #1211 from erikvold/717062v2

Bug 717062 Redux - Root preferences branch for simple-prefs is now configurable r=@Mossop