Add thorough acceptance/feature spec for playing stones

Also DRY up/refactor the remainder of gameplay_actions_spec
Improve method for updating game association cache after a turn

Also fix bug in game.pass method (currently wasn't causing problems)
Add feature specs and model unit specs for undoing a move

Also make superficial tweaks/cleanups to some existing tests
Add javascript and assets for creating modal dialogs
Implement undo move feature, including approval from opponent
Merge branch 'add-move-undo'