Fix movement rules in backgammon

[ktegan]

The current backgammon implementation doesn't perfectly handle the corner cases of what is and is not a legal move. To really do this right I think you have to look at all current moves and all subsequent moves.

# Backgammon movement rules from bkgm.com: A player must use both numbers of a roll if
#   this is legally possible (or all four numbers of a double). When only one number can
#   be played, the player must play that number. Or if either number can be played but not
#   both, the player must play the larger one. When neither number can be used, the player
#   loses his turn.  In the case of doubles, when all four numbers cannot be played, the
#   player must play as many numbers as he can.

I have added a number of unit tests (such as test_forced_moves()) in my branch and also made refactored the calculation of legal moves to be correct and more amenable to vector based calculation. Some of the changes in my branch are related to other things so I don't unfortunately have a completely isolated set of commits. Please see my branch at: https://github.com/ktegan/pgx/commits/kegan-backgammon