Exercise syncbase timing using beginEvent

In SMIL begin and end attributes, animations can have their timing depend on other
animations starting by using id.beginEvent or id.begin.

The difference is that id.begin is synchronous, while id.beginEvent
waits until the main thread is processing events.

See
http://www.w3.org/TR/SMIL20/smil-timing.html#Timing-EventSensitivity
http://www.w3.org/TR/SMIL20/smil-timing.html#q134
Merge pull request #71 from ewilligers/sync-on-beginEvent

Exercise syncbase timing using beginEvent