Added retina stylesheet support to module loading, style sheets were being generated but not loading
Merge pull request #972 from learndot/retina_modules

 Added retina stylesheet support to module loading
Adds minor comments tidy.
Removes private unused SC.View.prototype.prepareContext method.  As the comments pointed out you should not use this method it is OLD.
Adds debug mode only warnings if invokeOnce or invokeLast are called outside of a run loop.

I encountered a weird bug while working in the console due to the fact that invokeOnce was running after invokeNext, which will never happen in the context of the run loop.  A warning should help save developers from making similar mistakes.
Removes extra observers on SC.ButtonView.
merged edge