Fix Game.pass & Game.undo bugs, add unit tests

Refactor the tracking of captured stones across moves
Clean up some other Game model instance methods a bit
Enhance and clean up integration/feature specs

Make integration tests are more thorough (provide better coverage)
Refactor a fair amount as well -- more DRY and readable
Quite painfully overcame many obscure Postgres/Capybara errors >_<
Also do small tweak to .gitignore