Add Memcache cache method

Allows you to use memcache storage method for data, ie for rate limiting storage. Useful for multi-server setups where file or APC based storage will not work (APC is phased out of PHP 5.5 anyway). To use it, add to index.php:

    Defaults::$cacheClass = 'MemcacheCache';
    Defaults::$memcacheServer = '127.0.0.1';
    Defaults::$memcachePort = 11422;

I was thinking of moving cache classes into their own folder but will leave that decision up to maintainer.