Ottava Clefs (try 2)

8va Treble clefs added. See Request #236 for background -- There are still about 7 too-many commits here, but the net result is 4 files changed in a manner that should be easier to see. 
This might get unwieldly soon (since I'm thinking of adding `pitch_shift` too.) How about changing the last parameter to an `options` hash, and making `octave_shift` an option. (You can use `Vex.Merge` for setting defaults -- there are examples sprinkled across the codebase.)