demo of content negotiation and rest handling

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #202
| License       | MIT
| Doc PR        | -

This is to demo the _format operations in a simplistic case. Do we need anything more here, document somehow why or how we do it? http://symfony.com/doc/master/cmf/bundles/content/exposing_content_via_rest.html seems to pretty much cover it.

<rant-on>
I spent most of the time figuring out how to make the jms serializer work as expected. the doc is extremly vague as to how to overwrite configuration from other bundles, and how to name files and such things. this is what finally worked, after reading a bunch of random stackoverflow questions and trying things out.
</rant-on>
the [doc](http://symfony.com/doc/master/cmf/bundles/content/exposing_content_via_rest.html) says to say `prefer_extension: false` but this leads to having .json as extension and still get html, because the web browser prefers html over json.