Elfinder

See https://github.com/symfony-cmf/MediaBundle/issues/28 for installation to be able to test.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #28
| License       | MIT
| Doc PR        | TODO

**TODO to merge**
- [ ] code review / cleanup
- [ ] check root creating (see PHPCRDriver:: _stat)
- [ ] check rename error (DocumentManager::move results in an error)
- [ ] add installation documentation