[WIP] Added slideshow block and CmfMedia

***Do not merge, the source branch of this PR needs to be changed to ``prepare-next-version``.***

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 these PRs - they need to be merged first, before this PR can be merged:
  - https://github.com/symfony-cmf/MediaBundle/pull/5 - 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

**TODO to finish**:

- [x] rebase this PR on the prepare-next-release branch - the related block bundle PR (symfony-cmf/BlockBundle#72) must be rebased first
- [x] update cmf_block.imagine.multilang_slideshow_admin to updated block
- [x] update create image config
- [x] close this PR and create a new one setting the branch source to the prepare-next-release branch