Add seekCompleteCallback to seekTo.

I had created an audio sprite elsewhere:

    var sprite = new Media(/* omitted */);
    sprite.play();
    sprite.pause();

Then, further along, went to play the clip:

    sprite.seekTo( track.startTime );
    sprite.play();

In iOS 7 and below, as well as Android, this worked. However, in iOS 8, nothing would be played. When I inspected the position, `spirte._position`, a value of `-1` was shown. Basically, seeking to a position took some undetermined amount of time, and the audio couldn't be played until the seek had completed.

This pull request adds a callback to the `seekTo` method that is fired when the seek is complete.