[WIP] Code revamp.

- [x] Use connection, do not rely on entity manager.
- [x] Remove entity/model interface.
- [ ] Make all configuration optional.
- [ ] Separate Repository and Manager.
- [ ] Do not mix business in model.
- [x] Use `ManagerRegistry` to replace the current blog manager.

#### Demo application

https://github.com/kayue/symfony-wordpress-edition

#### Multisite usage

Before:

```php
$anotherBlog = $blogManager->findBlogById(2)->getEntityManager();
$posts = $anotherBlog->getRepository('KayueWordpressBundle:Post')->findOneById(1);
```

After:

```php
$manager = $this->get('kayue_wordpress')->getManager(2);
$post = $manager->getRepository('KayueWordpressBundle:Post')->findOneById(1);
```
Extra line.