Track image source, in case it is larger then Canvas view

Hi Matthieu, 

this pr is about [#13](https://github.com/MattKetmo/darkroomjs/issues/13). It performs crop using original image when the canvas was down scaled to `maxWidth/maxHeight`. So now it is possible to scale canvas to fit the container, but origin area size remains after cropping.

Or do you have other ideas to the subject?