Update all tutorials and ZopeSkel references for Plone 4.3

Now that 4.3 ships with ZopeSkel out of the box (thanks @smcmahon for very hard work) we need to update all tutorials to reflect this reality and get rid of the fragile old boilerplate.
- Check which zopeskel version 4.3 comes with
- Standardize content creation to this zopeskel version
- Standardize theme creation to this zopeskel version (which is actually Diazo compatible theme package, is such needed anymore?)
- Update instructions how to export Dexterity content types to this version
- Update all tutorials or add _Dragons_ warnings if they cannot be updated and are no longer compatible

@svx could you help with this task?

@zupo what are the implications of todoapp tutorial? Last time I checked it used a custom buildout, but I would prefer to see a solution where we start from the scratch with vanilla Plone installation.

:beers: for everyone.