Rails cache with splitting into chunks

This patch allows to store in the memcache very big arrays. We can store in the memcache only objects less then 1Mb, I've implement Rails.cache.fetch_splitted method which splits array in to chunks and store all data separately.

Rails.cache.fetch_splitted('Users.all', :split => 1000) do
  users = Users.all
end

I've just added write_splitted, read_splitted and fetch_splitted to the Rails::Cache::Store, so you can use splitted methods for all kind of storages.