Extended Callback: Now the compare callback will also return the origina...

Extended Callback: Now the compare callback will also return the original as well as the compareTo filename so it can be used in the callback method in order to tell the user which files had problems or were not equal to each other.

As an example:

```js
gm.compare('/path/to/image1.jpg', '/path/to/another.png', function (err, isEqual, equality, raw, orig, compareTo) {
 //do stuff

if (!isEqual) {
    console.log('PROBLEM: Unequal images found: %s with %s',orig, compareTo);
}
 //do stuff
})
})
```