use 'osv' module, supports os.tmpdir() on node v0.6

solves a [problem several people seem to have](https://encrypted.google.com/search?hl=en&q=%22has%20no%20method%20%27tmpDir%27%22):

problem, on node v0.6

    TypeError: Object # has no method 'tmpDir'

solution: use ['osenv' module](https://npmjs.org/package/osenv)