Improved XML support and enabling improves

#### Improved XML support

Before this PR:
````xml
<config xmlns="http://example.org/schema/dic/knp_rad">
    <!-- ... -->
</config>
````

After:
````xml
<config xmlns="http://knplabs.com/schema/dic/rad">
    <!-- ... -->
</config>
````

I did this change a month ago for the KnpMenuBundle too.

#### Enabling improves

Using [`Extension::isConfigEnabled`](https://github.com/symfony/DependencyInjection/blob/master/Extension/Extension.php#L118-125) allows configs using parameters, for instance:
````yaml
knp_rad:
    flashes:
        enabled: "%kernel.debug%"
````