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 requires the following changes:
https://github.com/schmittjoh/serializer/pull/184 (released in 0.14.0)
https://github.com/schmittjoh/JMSSerializerBundle/pull/340 (released in 0.13.0)
https://github.com/doctrine/phpcr-odm/pull/378 (released in 1.0.1)
https://github.com/doctrine/phpcr-odm/pull/380 (released in 1.0.1)

this is not required but improves the output structure:
https://github.com/symfony-cmf/ContentBundle/pull/91 (merged into 1.1-dev)