added a WP_Feed_Cache_Memcache client using memcache as backend

Hi,

Some background on this change. So we have a multiple server setup -- one backend server and couple of frontend servers all with wordpress installed. To make it secure, only backend server can write to wordpress database and all the frontend servers are read-only. And wp-admin etc has been removed from all the frontend servers. 

This setup runs fine in most of the cases (wp-cron is an exception but we have disabled it), until we added the RSS widget to sidebar. It seems the widget keeps last access time in wp_options table but in our setup frontend server cannot write to database:

```
WordPress database error UPDATE command denied to user 'blogro'@'12.34.56.78' for table 'wp_options' for query UPDATE `wp_options` SET `option_value` = ...
```

So I have modified the WP_Feed_Cache a little bit, when MEMCACHE_SERVER is defined, use WP_Feed_Cache_Memcache to cache the RSS last access time in memcache instead of DB.