[RFC] refactor resource resolver to resolve resource only if present as an action argument

https://github.com/KnpLabs/KnpRadBundle/wiki/Resource-Resolvers

This is something similar to ParamConverters.
It's however far more flexible, and accessible for any request that has a `_resources` attribute.

associated routing:

``` yaml

App:Test:
    defaults:
        _resources:
            blogPosts:  {service: app.entity.blog_post_repository, method: findAll}
            blogPost:  {service: app.entity.blog_post_repository, method: find, arguments: [name: id]}
            session:  {service: service_container, method: get, arguments: [value: session]}

```

and controller:

``` php

class TestController
{
    public function indexAction()
    {
        (var_dump(func_get_args()));
    }

    public function showAction(array $blogPosts, BlogPost $blogPost, Session $session = null)
    {
        (var_dump(func_get_args()));
    }
}

```