[WIP] Twig entity embed

Here's the corresponding d.o issue. https://drupal.org/node/2279257

It is built on 8.x-1.x right now instead of the embed-display branch. This PR will probably stay Work-In-Progress until it uses  EntityEmbedDisplay.
I wonder if we should tweak these parameters a bit. Put $entity_type (no default) first, then $entity_id_or_uuid, and finally $view_mode = 'default'. Have the second parameter be able to be either a UUID or an entity ID.