Add ability to save graph as PNG

[kittsville]

It'd be nice if it was possible to export portions of or the entire roo graph.

I quickly made this code to do it but it's not ready for proper use in Pyroon. When further essential stuff is implemented I might add this feature:

function saveAsPNG() {
	var image = new Image(),
	newWindow = window.open();
	image.src = cy.png({full:true,bg:'#FFF'});
	newWindow.document.write(image.outerHTML);
}