Default sentinel attribute to nil, add testing

- Fixes #158, defaults the sentinel attribute to nil, and if found to be nil in redisio::sentinel and redisio::sentinel_enable, the previous default sentinel will be used. As a new behavior, if it has been explicitly set to an empty array(`[]`), no sentinel instances will be created.

- RE: #158, needed to test my adjustment to default the attribute to nil would produce the result I expected, so I added chefspec/rspec tests to ensure my changes converge and behave as expected. `libraries/matchers.rb` is required by chefspec for matching against resources that are not built in. As a sid e bonus, anyone doing unit testing that uses `redisio` can benefit from these matchers as well.

- Added a Rakefile (and required Gemfile entries) to be able to run `rake style` (rubocop/foodcritic), `rake spec` (chefspec), or `rake integration` (test-kitchen). I mostly did this because `rspec test/unit` is more to remember and type vs `rake spec`.