added Crafty.pixelart

fixes #330 . However this is not working on FF (see original issue)

__Updated Example__
Example test that should work for all browsers:
```javascript
	Crafty.init(600, 300, document.getElementById('game'));

	Crafty.background('rgb(0,0,0)');
	Crafty.viewport.scale(6);

	Crafty.canvas.init();
	Crafty.pixelart(true);

	/*
	// Not working on FF
	Crafty.e("2D, Canvas, Image")
		.image("squares.gif");
	*/

	/*
	// Not working on Webkit
	Crafty.e("2D, DOM, Image")
		.image("squares.gif");
	*/

	//Should work cross-browser
	Crafty.sprite(50, 50, "squares.gif", {squares:[0,0]});
	Crafty.e("2D, Canvas, squares");
```