Add info about the sweet REPL in the README

Describe how you use the REPL, how to change the path of the socket,
and lists commands you can use in the REPL
add nc as possible client
add help to REPL and update README
make REPL location configurable with options

Allow `repl` config option passed into clusterMaster constructor

* `repl` - where to have REPL listen, defaults to `env.CLUSTER_MASTER_REPL` || 'cluster-master-socket'
  * if `repl` is null or false - REPL is disabled and will not be started
  * if `repl` is string path - REPL will listen on unix domain socket to this path
  * if `repl` is an integer port - REPL will listen on TCP 0.0.0.0:port
  * if `repl` is an object with `address` and `port`, then REPL will listen on TCP address:PORT
provide repl config examples
change REPL quit to stop

Since quit is a common command to exit certain
types of command shells, I felt it would be better
to use a different name for stopping the cluster,
since it would be easy to get confused and accidentally
shut down the cluster when simply wanting to exit the
REPL shell.

I switched the REPL command to stop() for a graceful
shutdown and a kill() for stopping hard.
add .exit to list of REPL commands

Although Control-D can be used, for some users it would
be good for them to know that they can also just type
`.exit`
v0.2.0