Created test for model referencing and maximum recursion depth on repr() call

[SteveGZr]

I added a test which fails when repr() is called on models which reference themselves over other models. Because this results in a "maximum recursion depth" error.

To solve this problem it is needed, that the custom repr() functions are deleted and the built-in is used.

[SteveGZr]

Is there a chance that you are going to merge this pull request? I still haven't found a good reason for keeping those custom repr() functions..