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.
In v0.2 code, a directory `test/output` was used for output. I could have added this directory to .gitignore or keep the `mkdtemp` approach used previously. I picked the latter option, because it seemed the cleaner one.