Ability to skip links based on criteria

This is based on discussions in https://github.com/TheFootballSocialClub/FSCHateoasBundle/issues/34 and expands on the PR https://github.com/TheFootballSocialClub/FSCHateoasBundle/pull/36 by @marapper.

I have squashed his 2 commits into one. I have changed the annotation to `skipIf`, the yml configuration value is `skip_if`. It now is property path and a value. Example: 

```php
/**
 * @Rest\Relation(
 *      "parent", 
 *      href = @Rest\Route("api_post_get", parameters = { "id" = ".parent.id"}), 
 *      skipIf = { ".parent" = null } 
 * )
 */
```

or

```yml
relations:
    - rel: parent
      href:
          route: api_post_get
          parameters: { id: .parent.id }
      skip_if:
          ".parent": ~
```