Rename Timeline to AnimationTimeline

Rename Timeline to AnimationTimeline
(From Web Animations minutes 27 Feb 2014
https://etherpad.mozilla.org/ep/pad/view/ro.WCBLOuse6D5/latest )

Introduce deprecation reporting, where a deprecated feature continues to
work for three months after deprecation, but a warning is logged to the
console when the feature is first used.

Illustrate how the deprecation reporting system is used, using the
obsolete Timeline constructor.