Implement multi_set method

[ryanc-bs]

Hi, I noticed that while there is a multi_get method for retrieving a batch of key/value pairs in a single request, there is no corresponding multi_set method for writing a batch into the cache. It seems that other clients such as python-memcache and pymemcached support writing batches of multiple values in a single request, so I was wondering if there is any particular reason why it is missing here?