Feature/allow use of mock redis

I wanted to use MockRedis in our test environment.

MinuteMan was checking the class of the passed in Redis object and instantiating a Redis object if itwast of type Redis

Not specifying this allows for passing in mock objects and pottentially other data stores.

This pr still accepts a hash of options, instantiates a new redis client if none was passed or simply uses what the programmer specifies

I had to specify a redis var on the Minuteman class in the test helper. 

I think the specified redis is not being used in operation.rb as that is a class level var? But in all honesty I havent had much time to test that theory. and it seems like a different issue to my use case.

PS, Thanks for the awesome gem! This has given us some great insight at work!