Added slideshow block and CmfMedia

Can be used to test CmfMedia

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | na
| Fixed tickets | na
| License       | MIT
| Doc PR        | https://github.com/symfony-cmf/symfony-cmf-docs/pull/191

To test:
- checkout this PR
- add the MediaBundle and LiipImagineBundle to composer (dev-master)
```
composer require liip/imagine-bundle symfony-cmf/media-bundle
```
- checkout these PRs - they need to be merged first, before this PR can be merged:
  - https://github.com/symfony-cmf/MediaBundle/pull/27
  - 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

- checkout the following packages to the latest master:

  - phpcr/phpcr-utils
  - sonata-project/admin-bundle
  - sonata-project/block-bundle

- run ``app/console doctrine:phpcr:repository:init`` - creates /cms/media