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?
Instead of having this in the game model, what do you think about moving it into the piece model?

```
piece = Piece.find(1)
piece.is_move_allowed?(5, 5)
```