Add xml support; add templating helper

This PR adds some XML support to this bundle. The namespace url became `http://sonata-project.com/schema/dic/bock`, this can be changed if you prefer something else.

Currenty, this bundle provides only a twig extension. This PR adds a Templating helper to use in PHP templates (I see I didn't push that to github).

At last, this PR removed some unused code from the twig extension.