List and Show choice type

https://github.com/sonata-project/SonataAdminBundle/issues/1023

This PR addresses this issue. It adds a new "choice" type to list and show views, that accepts a "choices" array and an optional "catalogue" for translations.

While this same behavior can be implemented with custom templates, these tend to be less reusable. On the other hand, using custom entity methods doesn't allow access to services, which limits it to static sets of options. This implementation also has the advantage of mimicking the default "choice" field type, which will help new users.

I also added Symfony/Yaml to the require-dev, as the existing tests seem to require it.
There's a matching PR in SonataDoctrineORMBundle that need to be merged at the same time for this to work. No BC should occur AFAIK.