Allow NPM packages to be installed from a URL

Update the NPM integration module to allow installing an npm package from a private URL instead of limiting it to public npm modules only.

This replaces the _isGitHubTarball() function to handle not just git tarballs, but to handle any compressed package on any server.

You then use this the same way as the current github tarball feature:
Inside your package.js file add:

```javascript
Npm.depends({
  'your-package-name': 'http://website.com/example/your-package-name-0.0.1.tar.gz'
});
```