Added multiplexer.go and multiplexer_test.go

Created the files multiplexer.go and multiplexer_test.go in the network package.  The multiplexer.go uses NetworkMessage and NetworkMessageHandler and has an in channel and out channel.  Currently all methods except for Connect and Listen have been implemented.  multiplexer.go differs from simplemultiplexer.go in that it stores slices of Hosts in a map that is organized by SwarmIds and the multiplexer_test.go test to make sure that the map structures of Hosts is properly implemented.