add support for xz compression

This adds xz as a compression option.
xz is alread supported by bsdtar without the external xz program.
use xz compression when pushing images to the registry

This makes the xz compression the default.
Merge remote-tracking branch 'unclejack/194-high-image-compression'
Fix a bug which caused 'docker push' to fail with 411-lenght-required in Go 1.0