HGETALL/INCR/INCRBY/DECR/DECRBY

Added zipstr handler for k,v,k2,kv to k=>v,k2=>v2 as well as
implemented int based incr and decr commands.
Re allow for empty keys.

According to @antirez, an empty (zero length key) is valid and
should be allowed, so allow it as he probably knows :)