Update for D8 changes

Hi

Had to fix a few things to make it work again:

- entity info is now EntityTypeInterface
- SystemListing is gone, used a directory iterator instead similar to ExtensionDiscovery, just simpler, as we have no need for all the blacklisting and stuff IMHO
- Update the example to use target_id for the type, otherwise it saved an empty bundle.
- Directly save the entity instead of using $resource->post(). That does two things that we IMHO don't want to do here, a) permission checks (this should make the log in as admin part in the tests unecessary) and b) validation, which isn't of much use to us I think and we could still add it manually.

Test is still blocked on https://drupal.org/node/2135573.