Added slideshow block and CmfMedia

Can be used to test CmfMedia

To test:
- checkout this PR
- add the MediaBundle and LiipImagineBundle to composer (dev-master)
```
composer require liip/imagine-bundle symfony-cmf/media-bundle
```
- checkout underneath PRs

These PRs need to be merged first:
- https://github.com/symfony-cmf/MediaBundle/pull/5 - merged
- https://github.com/symfony-cmf/BlockBundle/pull/72
- https://github.com/doctrine/phpcr-odm/pull/293
- https://github.com/doctrine/DoctrinePHPCRBundle/pull/72
- https://github.com/symfony-cmf/CreateBundle/pull/60