ENH + DOC: SoundPyo start + stop times; closes #271

- add parameters start and stop to SoundPyo (for files only)
- explored start only (infer stop == start + secs) to avoid param stop, but has issues:
  want a default value for sec for tones, but default for files should be play to the end of the file
- new test:  py.test tests/test_sound/test_sound_pyo.py
- added SoundPyo to API doc to document its features