Feature Page component

This adds support for a `Page` entity, which should be a minimal CMS implementation.

Currently a single `RouteLoader` is available and should be defined in your `routing.yml` as follows:
```yml
my_cms_route_name:
    resource: .
    type: elcodi_page_render
```

Everything else is made configurable by the `elcodi_page` key.
```yml
elcodi_page:
    mapping: ~ # usual mapping setup
    routing:
        loader:      elcodi.core.page.router.simple_loader.loader
        route_name:  elcodi_page_render_view
        route_path: '/{path}'
        controller:  elcodi.core.page.controller.page:renderAction
```

TO DO:
- [ ] add `replace` entries in `composer.json`.
- [ ] ease route configuration.
- [ ] multiple implementations of `RouteLoader`.
- [ ] gather feedback for changes.
- [ ] adapt to [fix/abstract-extension](https://github.com/xphere-forks/elcodi/compare/fix/abstract-extension) if/when it is accepted.
No need to put the `304` code here, since `Response::isNotModified()` will do it automatically if the `If-modified-since` [header matches](http://symfony.com/doc/current/book/http_cache.html#validation-with-the-last-modified-header).
Also, if we want to use weak validation for "semantically equivalent" content, we could generate a simple `ETag` based on a hash of the content. We can discuss this feature on a RFC for adding it later. BTW it looks like the Response object already supports [weak validation](http://api.symfony.com/2.6/Symfony/Component/HttpFoundation/Response.html#method_setEtag)