New tests and fixes for tuplets and meter

- Almost all test scores for meter and tuplets are now incorporated into MIDI import tests
- New option: use multiple voices in MIDI import or not
- Several fixes

Screenshots of some test files:

meter_12-8 
https://docs.google.com/file/d/0B5alKuFoSol2UkI0N1ZSRGZYSWM/edit?usp=sharing
meter_central_long_note 
https://docs.google.com/file/d/0B5alKuFoSol2MjQ0d0JNRVNwZms/edit?usp=sharing
meter_central_long_rest 
https://docs.google.com/file/d/0B5alKuFoSol2U3pkU1UwQUF2cEE/edit?usp=sharing