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 above does work, but seems weird to me. 

i wanted the configuration in to app/Resources/CmfContentBundle/config/serializer instead of inside a bundle. i would have expected this to work even without configuration, its a convention to overwrite many things from bundles, like the twig templates for instance. however, even with explicit config i did not get it to work.

the other thing that made me rant was that the doc does not explicitly say what part of the class namespace is needed where (filename vs the fqn inside the yml). the doc is quite vague about that... can you confirm that:

1. there can only be one location where serializer looks for yml/xml files per namespace
2. there is one file per class, named after the sub-namespace and classname relative to the configuration
3. the main key in the config file has to be the FQN
4. the serializer looks for a file for each class in the tree, even if some classes have no file (doctrine stops looking for files as soon as there is one unmapped class in the inheritance tree. the behaviour of serializer seems appropriate to me, just want to be sure)

i can try to do a PR on the doc to improve the points i am confused by. so that others can be less confused, and that i can look it up again later as i am bound to forget the details again ;-)