Renames - AnimationGroup and AnimationSequence

Rename ParGroup to AnimationGroup
Rename SeqGroup to AnimationSequence

From Web Animations minutes, 27 Feb 2014
https://etherpad.mozilla.org/ep/pad/view/ro.WCBLOuse6D5/latest
Merge pull request #572 from ewilligers/ParSeqGroup

Renames - AnimationGroup and AnimationSequence