Added prefix and retina options

Firstly, Great addition to grunt.

I went ahead and added some quick things for better support when working with high resolution images; in addition to adding a prefix - in the event that you don't want to pollute the global namespace.

The `spriteWidth` and `spriteHeight` are for the `background-size` property, since @2 images are double the size and would not fit inline. I also removed any mention of @2 from the name, since it is most likely undesired.


I have the following as my hbs template, for an example of retina. Note it is intended to compile to stylus later.
```
@media
only screen and (-webkit-min-device-pixel-ratio: 2),
only screen and (   -moz-min-device-pixel-ratio: 2),
only screen and (   min--moz-device-pixel-ratio: 2),
only screen and (   -ms-min-device-pixel-ratio: 2),
only screen and (     -o-min-device-pixel-ratio: 2/1),
only screen and (        min-device-pixel-ratio: 2),
only screen and (                min-resolution: 192dpi),
only screen and (                min-resolution: 2dppx)

	{{#sprites}}
	.{{name}}
		background-image: url('{{../baseUrl}}{{../fileName}}')
		height: {{height}}px
		width: {{width}}px
		background-position: -{{x}}px -{{y}}px
		background-size: {{spriteWidth}}px {{spriteHeight}}px;
	{{/sprites}}

```

Additionally, here is my grunt config, to visualize my seperation

```
spritepacker: {
	basic: {
		options: {
			template: 'sprite.hbs',
			destCss: 'css/sprite.styl',
			baseUrl: 'img/',
			padding: 2,
			evenPixels: true
		},
		files: {
			'img/sprite.png': ['img/sprite/*.png', '!img/sprite/*@2.png']
		}
	},
	retina: {
		options: {
			template: 'sprite@2.hbs',
			destCss: 'css/sprite@2.styl',
			baseUrl: 'img/',
			padding: 2,
			evenPixels: true
		},
		files: {
			'img/sprite@2.png': ['img/sprite/*@2.png']
		}
	}
}
```

Hope you like the modifications.