I have not reviewed the code, but this looks like a worthwhile addition. Thanks for doing it!
I think the name should be changed from MeasureAttachedSpanner to BarAttachedSpanner. A measure is the interval between bar lines. The spanner is attached to the bar line. While it requires some work, I think it's worth making the change to be more clear in our terminology. Thanks, Carl https://codereview.appspot.com/571180043/
