Added bool Timeline::empty() method.

Mirrors std::container empty methods and is a useful counterpart
to the existing getNumItems(). Makes it easy to wait for animations
to complete before processing certain actions:
// e.g. in an update loop, don't process things if animating:
if( mSpecialTimeline.empty() )
{
 processSpecialItems();
}
Merge pull request #288 from sansumbrella/timeline_additions

Added bool Timeline::empty() method.