For Redis instances that require authentication, the current Redis store does not work.  So, this commit adds optional support for utilizing the Predis password option.  It will pick up the password from the config file and pass it along when it instantiates the Predis client.

The test simply checks that adding the password option doesn't break the constructor. Since Predis parameters end up as protected, we can't verify that the instance contains the password.

Add a check to the constructor of the new redis store to check if predis/predis is available, and throw a critical configuration error if not.