Also update best_tile when merging horizontally.
Fix game logic bug: 2208 to the right gave 0228 instead of 0048.
Improve weights thanks to the cross-entropy method.
Workaround weird assertion error when using luajit.
Smoothness: treat empty cells as 0.
Add more information to game dumps.
Artificially load a game from a dump.
Initial implementation of expectimax
More suitable depth values.

Reached 4096 in 8 mins
Rename num_free_cells to num_free_cells_normalized

num_free_cells() is needed for dynamic depth changes
Display worst best tile after multiple runs.