Support parameterized class names for subscribers

Parameters are not resolved in the `PaginatorConfigurationPass`. This PR fixes this, so we can use parameters instead of hard coded classnames for event subscribers:

    <parameters>
        <parameter key="acme.directory.subscriber.class">Acme\DemoBundle\Subscriber\PaginateDirectorySubscriber</parameter>
    </parameters>
    
    <services>
        <service id="acme.directory.subscriber" class="%acme.directory.subscriber.class%" scope="request">
            <tag name="knp_paginator.subscriber" />
        </service>
    </services>

If this gets merged, the [blog article][1] should be updated as well, since using parameters is considered best practice.

[1]: http://knplabs.com/blog/knp-paginator-reborn