batch_size for multi_get

Hey there,

I found it nice to have a :batch_size option also for the multi_get method.
If you are working with large rows with many columns or super columns you might get into trouble fetching those with multi_get.

The :batch_size option sets a limit to how many columns are fetched per key in a single query and like the get_range_batch method loops until all possible columns or at least :count columns have been optained.

However, due to the fact, that always up to :batch_size columns are fetched per query and key, you might get back up to :batch_size * num_keys - 1 more columns than you specified with :count.

I've done only minimal testing to this feature and cannot guarantee that it always works as it should. But I want it to share, so others might use it as well.

Greetings,
FewKinG.