lyrics.py: fix regexes used by strip_cruft (make them case insensitive)

strip_cruft() should now correctly replace all <br> with \n thus making
insert_line_feeds() and sanitize_lyrics() functions superfluous (they have been
removed).
lyrics_sources: put sources in their correct category
test_lyrics.py: improve lyrics module coverage
remove a log.info()
Merge pull request #977 from KraYmer/lyrics_test

Lyrics test