adding resize canvas function for viewer


I wanted to resize the canvas when size of div or size of window changes.

EaselJS does not seem to have resize canvas function so it touches canvas attribute directly.

http://www.fabiobiondi.com/blog/2012/08/createjs-and-html5-canvas-resize-fullscreen-and-liquid-layouts/

@rctoris Is the width and height variable in Viewer meaningful? It seems to be used just for storing purpose.