Function to determine if two pngs differ

I would find it useful to be able to determine wether two pngs differ or not. Something like this:

``` javascript
PNGDiff.differ(streamA, streamB, function (err, differ) {
if (err) throw err

console.log(differ ? 'They are not a like' : 'They look exactly the same')
})
```

Start at the PNGDiff API and inspect how PNG streams are currently read and compared. Add the requested differ callback behavior for two input streams, then verify that identical and different PNGs produce the expected result and callback errors are handled.

Written by the indexing model from the issue text.