Add a function for playing as the AI

You have to run it through the REPL.

$ cabal repl
*Hs2048 System.Random> import System.Random
*Hs2048 System.Random> plai (set (empty 4 4) (Just 2) (0, 0), mkStdGen
32)
Score: 8568
8   2   4   2
64  32  16  4
128  64  32  16
512 256 128  32
Also consider number of available moves

This brings the top score to around 15,000.

Score: 14836
2    4   512   2
16   32   16    8
32   128 1024   2
2    8   64   16
Also consider the number of empty points

This doesn't change the maximum score but it increases the average.
Also consider the number of duplicate tiles

Again, this doesn't improve the maximum.
Fix tests