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
Earlier on in the lifetime of this bug, Sam Joch landed a patch to b2g's UpdatePrompt.js which writes the deviceinfo.previous_os setting: https://github.com/mozilla/gecko-dev/blame/master/b2g/components/UpdatePrompt.js#L406
But you are right, we need to check the previous_os/os settings first and then check ftu.enabled.