Multiplexed Commands Del, MSet, SInter and SUnion

Implementing multiplexed commands (Del, MSet, SInter and SUnion as well as SAdd, which is needed to test SInter and SUnion).

Tests with server done.
I don't get this loop. 
- If you come upon a MultiBulkReply thats empty, you discard everything and return a new empty "combined" reply
- otherwise you remove all Replies from base that are not in the Reply you are iterating on

I think the original intention here was to create a single MultiBulkReply for the MultiBulkReply objects of the child SInters - how is this fulfilling that?