2 2 stable

This ports the plugin to spree-2-2-stable.

Note that I've added the version in the Versionfile; which points to a branch '2-2-stable'. If you want to merge this PR, I think it is best to merge this into a '2-2-stable' branch. 

As mentioned in the tickets, I'm cleaning up the assets. But that is something that could be applied to master (2-1-stable) and 2-2-stable regardless of this port. So i'll make that a separate PR on master.