[POC] Metadata and Annotation support

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no (well, yes, but should be BC when finished)
| Deprecations? | yes
| Tests pass?   | ?
| Fixed tickets | 
| License       | MIT
| Doc PR        | not yet

This PR adds initial support for Annotations:

````php
use Symfony\Cmf\Bundle\RoutingBundle\Metadata\Annotations as CmfRouting;

/**
 * @PHPCRODM\Document(referenceable=true)
 *
 * @CmfRouting\Template("TestBundle:Content:index.html.twig")
 * @CmfRouting\Controller("cmf_content.controller:indexAction")
 */
class AnnotatedContent
````

For this I have introduced the `jms/metadata` package.

To maintain BC we would need to implement a DIC driver for the metadata factory.

Things to do:

- Add an XML driver (and I guess a YAML driver)
- Add the DIC driver to maintain BC