extract oriented? in Jhead worker
split worker argument arrays into lines, use long names were possible
use shorter url for advancecomp source in travis build script
specify pngquant version to compile explicitly in travis build script
use curl and pipe to tar instead of wget in travis build script
use pnquatn 2.3.0 in travis build script
Merge branch 'travis-fix'
`:skip_missing_workers` option to skip workers with missing or problematic binaries, resolves #66