[ConfigManager] Allow to define filebrowser*Url via a closure

This PR solves #69 in a different way. Instead of propagated the current request attributes, we can now define a file browser closure:

``` php
$builder->add('field', 'ckeditor', array(
    'config' => array(
        'filebrowserBrowseHandler' => function (RouterInterface $router) use ($request) {
            // The router is always passed as argument.
            // Return your url according to the router/request or any other objects.
        },
));
```