Single tick delay when delay=0

Because `Future.delayed` will always delay to at least the next tick, animations with a delay of 0 don't actually begin animating until the next tick. You can see this with an animation like the following:

``` dart
foo.animate().fadeOut(duration: 0.ms)
```

In this case, the `foo` widget will display for a single frame because it renders the beginning of the animation (100% opacity) initially, then jumps to the end of the animation (and 0% opacity) the next tick.

This is solvable with a fix similar to the below in `Animate._restart`, but I'm not convinced it's worth the change. The problem is unlikely to manifest in any but the most esoteric situations (like the above), the expected behavior is a bit ambiguous (ie. the current implementation isn't clearly wrong), and having events like `onComplete` become synchronous in some cases could introduce new issues.

``` dart
void _restart() {
_delayed?.ignore();
_initController();
_updateValue();

if (widget.delay == Duration.zero) {
// this prevent a one tick flicker.
_play();
} else {
_delayed = Future.delayed(widget.delay, () => _play());
}
}
```

Similar to #152 I'm going to defer any action on this until there's real world reports.