Added valuation functions
Added a valuation bot.

With seed 5, and the coefficients of 5, 1, 2, 2:

Total num moves: 850
Move breakdown: {"down"=>72, "left"=>81, "up"=>354, "right"=>343}
Final score: 14048
Final board:
[16, 64, 256, 1024]
[4, 32, 64, 256]
[2, 16, 32, 64]
[64, 2, 8, 2]