Callbacks, options & transitions

Hi,

Love this plugin, but needed to extend it to add some basic functions like callbacks and transitions. I tried to keep it light, but it's up to you if you want to include it!

## Example usage:
### One second fade-in
```javascript
$("li img").unveil(300, {transition:1000});
```
### Callback
```javascript
$("li img").unveil(300, {callback:function(image){
  console.log($(image).attr('src'));
}});
```

These can naturally be used together.