[WIP] Add and use media browser

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | https://github.com/symfony-cmf/symfony-cmf-docs/pull/251

**Installation**
- checkout the branch of this PR https://github.com/symfony-cmf/CreateBundle/tree/media_browser
- checkout elfinder branch of the MediaBundle https://github.com/symfony-cmf/MediaBundle/tree/elfinder
- install ckeditor according the create documentation http://symfony.com/doc/master/cmf/bundles/create.html#using-ckeditor-instead
- follow installation instruction added in https://github.com/symfony-cmf/symfony-cmf-docs/pull/251

**TODO**

- [ ] add cookbook article about integrating CmfMedia with ckeditor and elfinder for both Create and admin forms (CKEditorBundle/ IvoryCKEditorBundle)