enhance Timeline

Add method getter and setter for timeline currentTime. That
encapsulates player time completely.

Add method rewind that calls each registered module update methods.

Refactor Chapters.update to expect timeline object.
Call Timeline.rewind on player ended.
refactor `logCurrentTime` default timeline.update listener
Call jQuery’s trigger method on wrapped Timeline instance instead of
dispatching it to the player.
adapt ACTIVE_CHAPTER_THRESHHOLD

0.3 is too long for the third chapter in the test file to be activated.
only absolute links in examples
Merge branch 'timeline_abstraction' of github.com:line-o/podlove-web-player into timeline_abstraction