Replace Preloader class with a resourceLoader that supports multiple types of resources.
 - Add ImageResource/JSONResource/ScriptResource
 - Use ResourceLoader to load image resources, instead of Preloader static class.
 - Add Backbone.js for its Backbone.Events utilities, and mix them in with the Resource class to trigger ready/failed events.
 - Rename resources.coffee to api.coffee, to disambiguate with new resource base class.
Make Screen.TEXTURES use ImageResource's instead of straight Image elements.
Make sure the screen overlay is rendered before map shadows, so the darkness isn't pixelated.
Merge branch 'jdd-resource-manager'