Add support of relative & network path for CmsPageRouter

This PR adds support of absolute path, relative path, network path and absolute url for `CmsPageRouter::generate()`.

This also allows with Twig :

```jinja
{{ path(page) }}  {# /absolute/path/to/url #}
{{ path(page, {}, true) }} {# relative/path/to/url #}
{{ url(page) }} {# http://sonata-project.org/absolute/url/to/url #}
{{ url(page, {}, true) }} {# //sonata-project.org/network/path/to/url #}
```
why not calling ``UrlGenerator::getRelativePath`` directly ?