build: create smaller build artifacts

check if xz exists on the host system and use if available, which makes
xz-tarballs available for distribution. also remove deps/zlib/contrib
since it's not in use -- shaves additional size from the tarballs.

also, slightly modify the .gitignore file to ignore these new archives.

PR-URL: https://github.com/iojs/io.js/pull/319


Reviewed-By: Rod Vagg <rod@vagg.org>