Fixes #3 Provide identification of a message hub

A simple application that generates a random identity and hosts
this identity using a zmq REP socket.
Fixes #4 Configure identity of a message hub

Add a command line option 'identity' that will be used if
the option is present.

Example Usage: 'messub --identity=MyIdentity'
#7 refactor main loop to use a channel to signal an error in order to exit
#7 reply to heartbeat message
#7 register endpoint locally upon receiving any message
Fixes #8 send and receive local messages