Jokers

Some of my favorite card games have a joker.  I started implementing a "joker" card.  I'm not happy with this so far, as I **hate** ```Card.new(nil, nil, 'Red')```

But it's still worth talking about, at least to see if this is worth moving forward. 
@joeletizia I just wanted to point out one thing, here:

https://github.com/darrencauthon/rubycards/commit/e6c424f5b59b87129220c5b00a0396a6a92d0e93

When I merged in the changes from master, tests failed... which forced me to update the code like you wanted.

I know, we can eye-ball these sorts of things and say, "Oh, I'm passing in this, and I'm checking against this... but I **love** having a test force to to explicitly state that. :smile: