Simplify mode handling and core module
Update markdown mode to new architecture
Don't show border for dark themes
Don't use <code> for default mode
Avoid frame flickering
Remove unused dependency
Inline frame styles
Remove Mode Controller
Use Views as mode handlers, starting with markdown
Revert to having mode set properly on model
Update default mode to Backbone.View
Get new JavaScript mode semi-functional
Remove unused dependency
Get new CoffeeScript mode semi-functional
Update iframes once loaded
Clean up
Update the mode architecture

 * Use Backbone views rather than simple functions.
 * Handle iframes more appropriately, updating contents
   once the frame is loaded, etc.