Adding "cache_namespace" key in "orm.em.options"

In reference to issue #15 

When you have many web applications in the same server, it is necessary to configure a suffix in the APC cache in order to avoid cache conficts between applications.

With this patch, you can configure it in this way:

        $app->register(new Dflydev\Silex\Provider\DoctrineOrm\DoctrineOrmServiceProvider, array(
            "orm.em.options" => array(
                "cache_namespace" => "appname_",
                "metadata_cache" => "apc",
                "query_cache"    => "apc",
                "mappings" => array(
                    array(),
                ),
            ),
        ));