ListField: removed default `None` value for the `items_types` parameter.

[avrahamshukron]

This will create more consistent code since item_types is required. Tests had to be modified a bit. Specifically tests which used Model class with circular reference to itself. It was easier to just extract the dummy class outside the function, in order to avoid import path hell.

This change breaks the current API, so if merged it should be into a new major version.

Fixes #96

[beregond]

@avrahamshukron nice, and you are right - therefore lets make version 3 :) before that I need to release some small improvements to branch 2.x and then we will do all BC breaking stuff (this and regexes too)

[avrahamshukron]

Sure, thanks! Please hold the release of 3.0 until the weekend, I might have some more small fixes to push.

[beregond]

Ok, no problem

[avrahamshukron]

I totally forgot about this... If it is still relevant you can absolutely merge this.