Support for separate output_dir, clean up tmpdirs

c4e9bef closes #7. Depending on what you have in mind, it may also close #22, as the changes needed in get_transcode_dir to support a separate output_dir mean that only the basename of data_dir is altered by the regex substitutions.

841e575 cleans up the tmpdirs left lying around when creating torrent files for new transcodes. (Normally I would have submitted these as separate pull requests, but their patches overlap, so I edited the description of my initial pull request to include this one, as well.)