Import updated 8.0.x branch

[tstoeckler]

I've used the instructions in the README.md to create a subsplit of the core directory in the lastest 8.0.x branch (as of this writing). I didn't create a pull request, because I assumed you would want to create a 8.0.x branch here as well, instead of importing the changes in the 8.x branch.

The branch is at https://github.com/tstoeckler/drupal-core/tree/8.0.x