Added "scrollThen" that can apply a scroll animation before another transition

Related to issue #76

Usage:

  this.transition(
    this.fromRoute('posts'),
    this.toRoute('post'),
    this.use('scrollThen', 'toLeft'),
    this.reverse('scrollThen', 'toRight', {duration: 100})
  );
Merge pull request #82 from jerel/master

Added "scrollThen" that can apply a scroll animation before transitions