Added complete callback

I am pretty sure this isn't the way to go about doing this, but I need to know when all of the images are processed, and this seems to do the trick - it makes it simply enough to resolve a promise in the callback in order to conclude a task. e.g.

```coffee
g.task "images", ["clean"], ->
  deferred = q.defer()

  g.src "images/**/*"
    .pipe g.p.imagemin {}, -> deferred.resolve()
    .pipe g.dest "web/images"
    .pipe g.reload()

  deferred.promise
```