Update StashBundle for 0.10.* support

This pull request:

* Updates the StashBundle to make use of the current API as provided by the 0.10.* release of Stash.
* Transforms the CacheService object into a more direct wrapper of the Pool class, much as the CacheRequestObject wraps the Item object (and thereby provides additional functionality like getItemIterator)
* Makes several minor bugfixes and functionality improvements regarding how the bundle integrates Stash into Symfony.
* Very, very slightly expands test coverage