[WIP] enabled json/xml support

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

the goal is to enable something like curl http://cmf.lo/app_dev.php/en -H Accept:application/json

this currently fails because the serializer does not yet support PHPCR ODM requiring changes both inside http://github.com/schmittjoh/serializer and http://github.com/schmittjoh/JMSSerializerBundle