Symfony native configuration support

[zerkalica]

I added option to configuration tree. Now supported native symfony configuration: yml or xml.

# app/config/config.yml
ddeboer_guzzle:
    service_builder:
        configuration:
            aws:
                params:
                    access_key: 12345
                    secret_key: abcd
            s3:
                class: Guzzle\Aws\S3\S3Client
                extends: aws
                params:
                    subdomain: michael

[mtdowling]

This looks pretty cool. :ship: