upgrade implementation from 1.x to 2.x

[rallek]

as far as I see, the images are at another place. Might be also some other issues are different.

[rallek]

https://github.com/zikula/core/blob/master/UPGRADE-2.0.md
Try it out and see if something else is needed.

What is my upgrade strategy if someone is upgrading from an early stage of 1.5 to a late stage of 2.0? Have to discus with Axel. Currently there is no upgrade code needed so no problem. But as soon as #15 is comming I may have this issue.

Could be a strategy to release 2.0.x only after freezing 1.5.x