Merge branch 'dev' at v1.3.2-alpha.

Update packages.

Fixed:
  Won't reject an entire song if it has no album art.
  Removed flawed concurrencyLimiter code, reintroduced async.js for that.

Other:
  Updated to use fluent-ffmpeg 2.0.0-rc2.
  Refactored auth and dev-auth modules.