Read AGGs using Gambit::ReadGame

Files with initial token #AGG are recognized as AGGs and read by ReadGame().
Also added several example AGGs to contrib/games/.

GetPayoffDeriv for AGG now fully implemented; gambit-liap should work.

StrategySupport::NewMixedStrategyProfile now correctly outputs the AGG
version when appropriate. With this fix, gambit-logit now works.