Merge audiotranscode v0.2 into cherrymusic (fixes #489)

This preserves the `starttime` parameter introduced in cherrymusic audiotranscode. There are also a few other, very minor changes from the original audiotranscode0.2 code.

All these changes should be apparent in diffs of  `audiotranscode/__init__.py` and `test/test_transcode.py`. I'll also add some commit comments here on github in the appropriate places.
If the exception is simply passed on, the whole `except` block is not necessary. The original v0.2 code had that changed to
```
except Exception as exc:
    raise exc
```

but I didn't see what re-raising would accomplish except changing the stack trace (and thus make is less useful IMO), so I left it out.