Hacking in support for dbal result cache

Refs #114

This is hacked-in support. Some of the code may be usable, but really this shows that some refactoring is needed so that cache support isn't limited to ORM only.

Please do not merge in this code, just submitting it so that others can see where modifications are needed and run with it. I pretty much have no clue re: the XSD changes, but I didn't get any errors out of Symfony2 or PHPStorm with my changes. Regardless, even the XSD changes need refactoring, as the cache type definitions should be moved into a reusable location in the schema imho (so that it's accessible by both dbal and orm namespaces).

Also, this has only been tested with Service type cache instance. I don't have the time to attempt this proper.