[COOK-2771] - Add support for XZ compression

Signed-off-by: Sean OMeara <someara@opscode.com>
Version bump to 0.8.0
Publish 0.8.0 Changelog
Version bump to 0.8.1 (for development)
Merge branch 'master' of github.com:opscode-cookbooks/ark

Conflicts:
	metadata.rb