Use board_t type
Use contant CPROB_THRESH_BASE
Move cache and search termination to score_tilechoose_node
New heuristic scoring based on monotonicity, number of merge moves and
empty cells.
Adaptive search depth limit.
Heuristic scoring:
- Take into account average value of tiles on board, thus forcing
  earlier merges of high tiles.
- Tune up individual parameters for better results - the chosen values
  are based on the results of CMA ES.
Merge pull request #27 from xificurk/ng

Improved algorithm reaching 32k tile