added optional percentage and image index for "each" function call

I needed this to show the preloader progress, also people are asking for this sometimes.

How to use: 

```
$.imgpreload([
    'http://lorempixel.com/1/1',
    'http://lorempixel.com/1/2',
    'http://lorempixel.com/1/3'
], {
    each: function (percentage, index) {
        console.log(percentage, index);
    }
});
```

Will return: 

```
33.33 0
66.67 1
100.00 2 
```

Also guys — http://www.lotterypost.com/js-compress.aspx — Microsoft minifier compresses the code just a little bit better :)

Cheers.