SlideshowBlock

This PR provides the possibility to create a slideshow block of child items. However the PR only cares for the structure of the slideshow, not the behaviour (otherwise we would lock the user in a certain JS library and/or jQuery plugin).
I'm not entirely sure if this should go into the BlockBundle since it's not a structural block (like ContainerBlock or ReferenceBlock) but a concrete one. Then again I wouldn't know where else to put it (reminds me on the discussion on the RssBlock).
I also wonder how we should render the images without adding a dependency on [LiipImagineBundle](https://github.com/liip/LiipImagineBundle) to the BlockBundle.

What do you guys think?