Bug 939174 - Launch whats new FTU tutorial to demo edge gestures etc.

* Add shared VersionHelper
* Unit tests for VersionHelper, FTULauncher
* Check and set device.previous_os in FTULauncher/FTU app
* Launch full ftu when ftu.enabled always
* Launch 'whats new' ftu tutorial when upgrade detected
* Add config for 1.3-2.0 and 1.4-2.0 upgrade tutorial
I feel that update previous version should be done in update_manager.js instead of writing the value each time the FTU is closed.