Factor out TimedItem._updateItemTime()

This is step 1 of 4 to make TimedItem._updateTimeMarkers() more readable.
Use an early-out to reduce indentation in TimedItem._updateTimeMarkers()

This is step 2 of 4 to make TimedItem._updateTimeMarkers() more readable.
Factor out TimedItem._updateAnimationTime()

This is step 3 of 4 to make TimedItem._updateTimeMarkers() more readable.
Factor out TimedItem._updateIterationParams()

This is step 4 of 4 to make TimedItem._updateTimeMarkers() more readable.
Merge pull request #73 from steveblock/updateTimeMarkers

Make TimedItem._updateTimeMarkers() more readable