one canvas for game board

Bug #3143 add back button to dialog boxes
Bug #3102 Auto-Save Functionality updated (FDD update ver. 0.23) (localStorage fallback to cookies)
Bug #3103 Saved Game requirements clarified
Bug #3305 new powerup animation animation
Bug #3125 [How to Play] Dialog is not according to FDD

I still don't love the way BoardBuildAnimation looks (seems a like an unneeded double-draw). I'm open to suggestions.

first release after the major image optimizations initiated by Leonid

• new map screen animations aren't implemented yet (they need to be reduced in dimensions)
• dialog buttons don't look right; need help understanding how to position new button images