download npm separately

This is a fix for https://github.com/oortcloud/heroku-buildpack-meteorite/issues/26.  With Meteor 0.6.6.2, Node 0.10.21 is required, but the run_npm function in bin/compile no longer works with $VENDORED_NODE/bin/npm. I copied the code from the latest version of [Heroku's Node buildpack](https://github.com/heroku/heroku-buildpack-nodejs), which downloads NPM separately from Node.