Add AI experiment option

The experiment mode tests 4 different AIs from 2 betting and 2 playing
strategies.
Refine AI betting strategy

The maximum bet is now 12 * the minimum, whereas previously it was a
function of money remaining.  The easy AI was made more deterministic
(easier to experiment with). The hard AI was made more aggressive, which
seems to improve its long-run performance in practice.
Clean code

Various spacing and other miscellaneous fixes.
Merge pull request #1 from vancezuo/ai-experiment

Merge ai-experiment into master