Added pawn move logic in Game model, and test

I wrote some code in the Game model that tests whether a pawn move is valid, and an accompanying test for it using FactoryGirl. 

The new "self.is_move_allowed? function in Game.rb first calls a helper function, one per piece type (I wrote one for pawns called "legit_moves_for_pawn"), which returns an array of x/y coordinates that a specific piece (in this case a pawn) can move to. (This could also be used to generate a list of highlighted squares indicating possible target destinations for a piece.) The "is_move_allowed?" function then tests whether the new [x, y] destination coordinates that the player has selected are within that returned array of valid coordinates.

Hopefully this is a good template for writing the piece move logic for other pieces. Thoughts?
Could this if statement be abstracted?  I feel like there's probably a way to use the same logic in both directions.

In the case of the pawn, it's not the end of the world, but figuring out a way to implement the logic only once will cut down about half of the work.

I'm not exactly sure how you could do it but you may be able to have something like:

```
direction = 1 # white
direction = -1 # black
end_y_position = 0 #black
end_y_position = 7 #white
```