Lyrics enhancements + unit tests

This PR finishes the job started by @paulp in PR https://github.com/sampsyo/beets/pull/582 :

- strip featuring artists from title and artist strings
- extend the dual songs handling of original PR by handling n songs combinations : iterate over the titles and append all found lyrics together  
- add unit tests (reusing some from abandoned PR https://github.com/sampsyo/beets/pull/386, hope I'll have time to push more tests in another PR later)
- clean up most of flake8 warnings (I would have corrected them all, but cannot make `tox` works on my mac yet so I had to parse the log on Travis)