Topics/improve memcache

Hey,

I hacked in some changes for the service provider.

We were primarily after injecting an already configured Memcache object.

And I made the `port` optional.

Before you use the service provider, you can setup Memcache with multiple servers like so:

```php
$memcache = new \Memcache;
$memcache->addServer('node1', 11211);
$memcache->addServer('node2', 11211);
$memcache->addServer('node3', 11211);

$app['orm.cache.factory.backing_memcache'] = $memcache;
```

Let me know if you guys have any suggestions.