Provide data migration from AT >> DX

[MrTango]

It would be nice for some users to have a way to migrate old Archetype based Newsletter data to Dexterity data.
Right now no migration exist, because of lag of founding.
There are two way one could go.

1. Use the 4.x branch which supports AT and DX to develop an upgrade step to migrate the data in-place.
2. Use collective.exportimport to export and import the data direktly from version 3.x to the current one.

If anyone needs this, i can help if there is a bit of founding available.