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 latest commit makes it non-breaking.

Since retina sprites have to play by different rules, this is required. I added a modified X and Y as well as sizes and the background position. 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.
```
{{#sprites}}.{{name}}
{{/sprites}}	background-size: {{rwidth}}px {{rheight}}px;

@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: {{rheight}}px
		width: {{rwidth}}px
		background-position: -{{rx}}px -{{ry}}px
	{{/sprites}}

```

Additionally, here is my grunt config, to visualize my seperation

```
spritepacker: {
	basic: {
		options: {
			template: 'sprite.hbs',
			destCss: 'css/sprite.styl',
			baseUrl: 'img/'
		},
		files: {
			'img/sprite.png': ['img/sprite/*.png', '!img/sprite/*@2.png']
		}
	},
	retina: {
		options: {
			template: 'sprite@2.hbs',
			destCss: 'css/sprite@2.styl',
			baseUrl: 'img/'
		},
		files: {
			'img/sprite@2.png': ['img/sprite/*@2.png']
		}
	}
}
```

Hope you like the modifications.