Allow to give a type of APC cache to clear

I've added a `type` configuration parameter in order to specify `apc_clear_cache(type)` argument.

This value can take one of these values: `all`, `user` or `system`.

See apc_clear_cache() function documentation: http://php.net/apc_clear_cache

Here is how the configuration will looks like:

```yml
sonata_cache:
    caches:
        apc:
                token:  s3cur3   # token used to clear the related cache
                type:   all      # cache type to clear (could be: all, user or system), default to user
                prefix: test     # prefix to ensure there is no clash between instances
                servers:
                    - { domain: kooqit.local, ip: 127.0.0.1, port: 80}
```