Display MusicBrainz URLs

Closes #574
Clarify debian/ubuntu pip installation.
Merge pull request #577 from johtso/show-musicbrainz-urls

Display MusicBrainz URLs
changelog/thanks for #577
add link to Slackware SlackBuild
changelog for #570
changelog (& tiny tweak) for #567
use travis_retry with apt-get commands

Avoid spurious failures on connectivity issues.
changelog for #579/#578
Merge pull request #580 from ian-kelling/master

Clarify debian/ubuntu pip installation.
Fix/enhance regexes related to lyrics searches.

The assignment of match.group(0) meant the entire string would be
assigned, i.e. it was a no-op even when it matched. The first match
is in match.group(1).

Strips the featured artist string from both artist and title
before searching. Also strips parenthesized addendums from the
title, such as (live), (acoustic), etc.
Teach lyrics plugin to look for dual song lyrics separately.