Add poker exercise

[ageron]

A couple things tripped me up:

• Ace, 2, 3, 4, 5 is the smallest straight
• 2, 2, Q, K, A is smaller than 3, 3, 4, 5, 6: you must compare the biggest groups first.

I started by implementing a function to compare two hands, but I found it easier to implement a function that just returns a hand overall rank, and then simply keep the hand(s) with the highest rank.

[isaacvando]

Very nice!