remove uses of String.Format to avoid problems with curly braces (see issue #1)
unify logging for client and server messages
Introduce low level MSet method and fix support for non-ASCII keys

- MSet fixes the implementation of Set (IDictionary<string,byte[])
- Set (IDictionary) calls the low level MSet (string [], byte [][])
- extend tests with a binary value and a non-ASCII key in MSET