Added imageSmoothingEnabled option on fabric.StaticCanvas

I would like to be able to set the `imageSmoothingEnabled` property on a canvas when I'm instantiating that canvas with Fabric and have Fabric take care of the various vendor prefixes for me. Image smoothing is often undesirable when rendering images with a 'pixel art' aesthetic.

I've created a fiddle that highlights my motivation for having this option: http://jsfiddle.net/LeJmQ/4/

As you can see the image on the right is much sharper.