Introduce ImageManager to load and render images

[clintbellanger]

Currently each class/object in Heroine Dusk basically handles its own image loading. This is a mess of course.

I want to add an ImageManager sort of class that loads all the image assets for the game, handles the loading callbacks, and maybe wraps image rendering to include proper scaling.

This class should be generic enough to be part of a reusable HTML5 game toolkit for future projects (along with files like input.js, loadbar.js, etc).