Migrate from Posts Bundle to Content Types bundle

Based on extensive conversations with @stevector and based on how much work new content types continue to be (as proven by @mavimo, @asm89, myself, and others) I've decided to further refactor PostsBundle pretty much out of existence. In its place is a kernel configured ContentTypesBundle.

Overall, the new configuration for `sculpin_content_types` looks like this:

```yaml
sculpin_content_types:

  posts:                  # exposes "posts" data provider
    type: path
    path: _posts
    layout: post
    permalink: blog/:year/:month/:day/:filename/
    taxonomies:
      - tags              # exposes "posts_tags" data provider
      - categories        # exposes "posts_categories" data provider

  talks:                  # exposes "talks" data provider
    path: _talks
                          # "type: path" is default
                          # "layout: talk" is default (singularize)
                          # no taxonomies by default

  projects:               # exposes "projects" data provider
    type: meta            # matches any source with "type: project" (singularize)
                          # "layout: project" is default (singularize)
    taxonomies:
      - tags              # exposes "projects_tags" data provider


  work_projects:          # exposes "work_projects" data provider
    type: meta
    meta: work            # matches any source with "type: work" metadata
```

The default configuration is this:

```yaml
sculpin_content_types:
    posts:
        type: path
        path: _posts
        permalink: pretty
        taxonomies:
            - tags
            - categories
```

This means that the posts are still enabled by default and they are still configured exactly the same way. The only difference is that rather than configuring it using post bundle specific configs they use a shared global content type configuration with other content types.

---

# BC BREAK

This is of course a huge BC break. PostsBundle hasn't been removed completely but it exists pretty much just to give the following nice exception if you try to run this patch with `sculpin_posts` in your `sculpin_kernel.yml`:

> [Symfony\Component\Config\Definition\Exception\InvalidConfigurationException]
> Posts are now configured in the 'sculpin_content_types' section of sculpin_kernel.yml, please see documentation on configurating content types

It isn't pretty but I hope it helps?

If posts were previously configured like this:

```yaml
sculpin_posts:
    layout: posts
    permalink: blog/:year/:month/:day/:filename/
```

... the configuration will need to be changed to:

```yaml
sculpin_content_types:
    posts
        layout: posts # this is the default layout value!
        permalink: blog/:year/:month/:day/:filename/
```

refs #95