User system http_proxy when it is available to use
dont use lazy streams for generating the release or archive files.

apparently using lazy streams for generating the release will cause the finish event to fire too early, which can often cause mac packaging to write only part of the destination file, creating an invalid zip.
Merge branch 'master' of github.com:leftstick/grunt-node-webkit-builder into leftstick-master
release 0.1.12