DATAREDIS-324 - Update Makefile to make it more idomatic.

Previously the Makefile for starting and stopping the Redis and
sentinel instances wasn't particularly idiomatic.  Rather than having
targets based on created files, it used a lot of .PHONY targets.  This
change makes the Makefile idiomatic for everything but stopping, and
the tests and points to the way of migrating it into a Gradle build,
eliminating the need for the Makefile altogether.

Applied @nebhale's patch.