build: add tar-headers target for headers-only tar

to replace the full src download by node-gyp, using the proper format
instead of the full source format

PR-URL: https://github.com/nodejs/io.js/pull/1975


Reviewed-By: William Blankenship <william.jblankenship@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>