Rethinking architecture

 - Main class is now a subclass Game instead of Window
 - Game replaces World
 - Declarative options with sane defaults (requires activesupport)
   - `window` and `asset_manifest` options on Game
   - `asset` option on SpriteEntity
 - DrawContext belongs to Window
 - Game#update takes secs_elapsed arg (like an Entity does)
 - Game#draw takes context arg (like an Entity does)