Rename TimedItem.specified to TimedItem.timing

TimedItem.specified is now deprecated. It has been replaced by
TimedItem.timing.

Meeting minutes describing this change:
http://lists.w3.org/Archives/Public/public-fx/2014AprJun/0005.html
Merge pull request #582 from ewilligers/DeprecateSpecified

Rename TimedItem.specified to TimedItem.timing