Implement score method for cards of type "Victory"

[evanofslack]

When calculating a player's score, we want to iterate through all of a player's cards and sum the points of each victory card. Each victory card should have a method score that returns an integer (number of VP that card scores).

Calculating VP for basic victory cards (Province, Duchy, Estate) is straightforward, but this method would be important for interactive victory cards (Gardens).