[WIP] Adding more dependency injection.

Here's the d.o issue https://drupal.org/node/2281491

As of the creation of this pull request there is only one commit that adds a create method. It isn't adding any benefit yet and it is a work in progress.
I think we're missing a declaration for the $this->entityManager class property.

Copied from EntityEmbedFilter:
```php
  /**
   * The entity manager service.
   *
   * @var \Drupal\Core\Entity\EntityManagerInterface
   */
  protected $entityManager;
```