Filter out empty source maps to prevent errors

This fixes plumberjs/plumber#12 and closes plumberjs/mercator#3 (invalid solution).
Update comment
Tidy up
Merge remote-tracking branch 'origin/master' into oja-empty-source-map-error

Conflicts:
	lib/util/supervisor.js
Log error if source map file is not found
Merge pull request #18 from OliverJAsh/oja-empty-source-map-error

Do not error if source map exists but is empty