Use the configuration class for the bundle config

This pull request:
- Makes it possible to split the sonata_seo configs over multiple config files
- Makes ``app/console config:dump-reference SonataSeoBundle`` work (fixes #59)

I tested with the config from http://sonata-project.org/bundles/seo/master/doc/reference/usage.html

AFAIK this pull request is BC but if that is not the case please let me know so I can fix that.